I have used the example program in http://msdn.microsoft.com/en-us/library/bb286958.aspx to list all of the users configured for access to a Team Foundation Server. It lists the users by the DisplayName property of the TeamFoundationIdentity class, which gives an kind-of-sort-of email-ish format which is hard to use to positively identify the users.
Is it possible to programatically get TFS to provide the users' login, the way they are shown when using Team Explorer to administer the TFS server?