When I execute the sp_helplogins 'login name' procedure, it displays the value MemberOf in the UserOrAlias column. What does this value mean?

As described in the sp_helplogins documentation (available at http://msdn.microsoft.com/library/default.asp?url=/library/en-us/tsqlref/ts_sp_help_50mr.asp), the UserOrAlias column can contain the value MemberOf, which means UserName is a role, or User, which means UserName is a user account. So, the login you're passing in is a role not a user account.