Your talking Remote desktop sharing, which only allows 1 active connection at a time. so do the users have local admin rights or network admin rights? There are groups that you can put people in so that they can remote into a machine even if they are not admins.
as to what you want, I'm not sure I understand. Can you explain a bit please?
do not want them calling another machine with RDS running and controlling it - just a specific user that they will not know the password of.