Many times during a migration to SBS 2008, users will be "left behind" and not shown in the SBS console.  If you add users "the old fashioned way" with the Active Directory Users and Groups console, they won’t show up in SBS.  I’ve written a tutorial on Spiceworks on how to resolve this situation.  Long-time SBS admins like me (and Susan) take this for granted but there are many more admins who are unfamiliar, or worse, trying to apply "enterprise" practices to SBS.