Issue:
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Although a user is already part of a TFS group, he can't access TFS work items (source code is fine), and TFS Web client/Visual Studio display the following message:
TF201072: A user or group could not be found. Verify that the users and groups used in your work item type defiition have been added to Team foundation server.
Workaround:
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The cause is related to some snag in the TFS identity synch service, and it is usually related to a specific collection. There are many articles on the web which help determine the root cause, so here we are only focusing on a fast workaround.
Attaching and re-attaching the collection will force a refresh of the identity synch data, and will usually resolve the issue.