Some time ago I blogged about the few lines of code required to convert a TFS Group to a Team. It's now available as part of the latest version of the Tfs Team Tools from codeplex.
D:\Sources\TfsTeams\Main\TfsTeams CmdLine\bin\Debug>tfsteams GroupToTeam /collection:http://localhost:8080/tfs/DefaultCollection /teamproject:Scrum /group:NoTeam Group [NoTeam] converted to team. D:\Sources\TfsTeams\Main\TfsTeams CmdLine\bin\Debug>
Grab it while it's hot :). This is especially handy if you've upgraded an older Project Collection to TFS 2012 and where you were already using groups to distinguish teams.