This is not possible with RDP and is a deliberate limitation imposed by Microsoft.
Terminal Services and MultiPoint Services require an additional license and to use it the way you propose would be getting around that license restriction. I'm not saying you're doing anything nefarious; this is just the way Microsoft sees it.
In order to do this, you will need a screen sharing application that can do meetings, such as TeamViewer, GoToMeeting, LogMeIn, etc. I know from experience that TeamViewer in particular works just fine from an RDP session and supports multiple people viewing the same session at the same time.
Otherwise, your only other option is session shadowing, but buddy1 and buddy2 must still have their own accounts so they can log in to shadow Admin's session, and they also must be local amins on the server itself.