It is a common issue for a lot of TFS Admins using the TFS Integration Tools to encounter an error message similar in nature to:
"…the migration service account ‘TFSService’ is not in the Service Accounts Group"

Thankfully there is a relatively easy solution, and the error could be avoided if this is a pre-migration step. TFS Admins will need to run a single command from the console on the TFS App Tier.

Please be aware, to use this command, you must have the required access rights to the TFS environment. Please see details http://msdn.microsoft.com/en-us/library/ms252504.aspx

General format of the command:
TFSSecurity /g+ groupIdentity memberIdentity [/collection:CollectionURL] [/server:ServerURL]

Example:
c:\Program Files\Microsoft Team Foundation Server 11.0\Tools\tfssecurity /g+ "Team Foundation Service Accounts" n:Domain\UserName ALLOW /server:http://TfsServer:8080/tfs

 

I certainly hope this helps, cheers!