<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2017-02-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000937098</issuerCik>
        <issuerName>TRINET GROUP INC</issuerName>
        <issuerTradingSymbol>TNET</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001694481</rptOwnerCik>
            <rptOwnerName>AGI-T, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ATAIROS MANAGEMENT, L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>40 MORRIS AVENUE</rptOwnerStreet2>
            <rptOwnerCity>BRYN MAWR</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19010</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001393014</rptOwnerCik>
            <rptOwnerName>Angelakis Michael J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ATAIROS MANAGEMENT, L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>40 MORRIS ROAD</rptOwnerStreet2>
            <rptOwnerCity>BRYN MAWR</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19010</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001671176</rptOwnerCik>
            <rptOwnerName>Atairos Partners GP, Inc.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ATAIROS MANAGEMENT, L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>40 MORRIS ROAD</rptOwnerStreet2>
            <rptOwnerCity>BRYN MAWR</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19010</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001671185</rptOwnerCik>
            <rptOwnerName>Atairos Partners, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ATAIROS MANAGEMENT, L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>40 MORRIS ROAD</rptOwnerStreet2>
            <rptOwnerCity>BRYN MAWR</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19010</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001671122</rptOwnerCik>
            <rptOwnerName>Atairos Group, Inc.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ATAIROS MANAGEMENT, L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>40 MORRIS ROAD</rptOwnerStreet2>
            <rptOwnerCity>BRYN MAWR</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19010</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001694477</rptOwnerCik>
            <rptOwnerName>A-T Holdings GP, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ATAIROS MANAGEMENT, L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>40 MORRIS AVENUE</rptOwnerStreet2>
            <rptOwnerCity>BRYN MAWR</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19010</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001697046</rptOwnerCik>
            <rptOwnerName>A-A SMA, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ATAIROS MANAGEMENT, L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>40 MORRIS AVENUE</rptOwnerStreet2>
            <rptOwnerCity>BRYN MAWR</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19010</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001697053</rptOwnerCik>
            <rptOwnerName>A-A SMA GP, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ATAIROS MANAGEMENT, L.P.</rptOwnerStreet1>
            <rptOwnerStreet2>40 MORRIS AVENUE</rptOwnerStreet2>
            <rptOwnerCity>BRYN MAWR</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19010</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2017-02-01</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>89</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>89</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>89</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">This Form 4 is being filed by more than one Reporting Person. The restricted stock units reported herein  were granted to Michael J. Angelakis in his capacity as a director of the Issuer.  A-T Holdings GP, LLC is the general partner of AGI-T, L.P. Atairos Group, Inc. is the sole member and manager of A-T Holdings GP, LLC and the sole limited partner of AGI-T, L.P. A-A SMA GP, LLC is the general partner of A-A SMA, L.P. Atairos Group, Inc. is the sole member and manager of A-A SMA GP, LLC and the sole limited partner of A-A SMA, L.P. Atairos Partners, L.P. is the sole voting shareholder of Atairos Group, Inc. Atairos Partners GP, Inc. is the general partner of Atairos Partners, L.P. Michael J. Angelakis directly or indirectly controls a majority of the voting power of Atairos Partners GP, Inc. and serves on the board of directors of the Issuer. Each Reporting Person disclaims beneficial ownership of the reported securities except to the extent of its or his pecuniary interest therein.</footnote>
        <footnote id="F2">Each Restricted Stock Unit represents a contingent right to receive one share of TNET common stock.</footnote>
        <footnote id="F3">The Restricted Stock Units will vest 100% on February 5, 2017. The Restricted Stock Units are also subject to accelerated vesting upon certain events.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>By: ATAIROS PARTNERS GP, INC., by /s/ David L. Caplan, Vice President</signatureName>
        <signatureDate>2017-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>By: ATAIROS PARTNERS, L.P., by ATAIROS PARTNERS GP, INC., its general partner, by /s/ David L. Caplan, Vice President</signatureName>
        <signatureDate>2017-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>By: ATAIROS GROUP, INC., by /s/ David L. Caplan, Vice President and General Counsel</signatureName>
        <signatureDate>2017-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>By: A-T HOLDINGS GP, LLC, by ATAIROS GROUP, INC., its sole member and manager, by /s/ David L. Caplan, Authorized Signatory</signatureName>
        <signatureDate>2017-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>By: AGI-T, L.P., by A-T HOLDINGS GP, LLC, its general partner, by ATAIROS GROUP, INC., its sole member and manager, by /s/ David L. Caplan, Authorized Signatory</signatureName>
        <signatureDate>2017-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>By: A-A SMA, L.P., by A-A SMA GP, LLC, its general partner, by ATAIROS GROUP, INC., its sole member and manager, by /s/ David L. Caplan, Authorized Signatory</signatureName>
        <signatureDate>2017-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>By: A-A SMA GP, LLC, by ATAIROS GROUP, INC., its sole member and manager, by /s/ David L. Caplan, Authorized Signatory</signatureName>
        <signatureDate>2017-02-03</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>By: /s/ Michael J. Angelakis</signatureName>
        <signatureDate>2017-02-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
