The code you are using has indeed exceeded capacity due to the strong interest in the hosted Team Foundation Service preview.
I would encourage you to sign up for our waiting list at
https://tfspreview.com/_InvitationCodes/CodeSignupRequest
Customers on the waiting list are processed in the order in which they are added, and once we have capacity we will send you a unique code that you can use to activate your preview account.
Technical Evangelist, Visual Studio Team System