When trying to map a local folder to a TFS server as a different user than the one under which I was logged in, I kept getting the message:

The working folder [source path] is already in use by the workspace [workspace name];[owner] on computer [tfs server].

I was finally able to map the folder by first deleting the local workspace as the logged in user:

tf workspace /delete /server:[tfs server] [workspace name];[owner]

Note that the "owner" was the currently logged-in user.

Thanks to the "Geek-A-Boo" blog for the solution at this page.