I forgot where I learned this (perhaps the TFS Admin guide) but ... this is a good little tip...

When you add users to groups in AD and those groups are present in the Team Foundation Server Security groups it can take up to 60 minutes for the policy to be pushed down into TFS.

To jumpstart this, restart the "Visual Studio Team Foundation Server Task Scheduler".

After the service restarts it kicks off the services that are responsible for refreshing the security policies. Now, if you add a user and they need immediate access, you can do it. Beware of other users though, this could impact your system if someone is checking in files, or working with TFS. So take caution in doing this.