The following instructions apply to Windows XP/2003/Vista/2008/7/2008 R2.
Please note that licensing for multiple active sessions on Windows is available only for the server editions (2003, 2008, 2008 R2).
For desktop licensed Windows'es (such as XP/Vista/7) you might wanna Google for how to force Windows to allow multiple RDP sessions, and make sure you don't do something illegal.
To have multiple people in the same session (as in they all control the same desktop, or watch another person doing something):
Open Local Group Policy Editor by typing gpedit.msc in run or cmd.
Enable "Set rules for remote control of Remote Desktop Services user sessions" in the Local Group Policy Editor, then reboot. The setting is under one of these paths depending on your OS:
Windows 2008/2008 R2
[Computer Configuration/Administrative Templates/Windows Components/Terminal Services/Terminal Server/Connections]
Windows Vista/7
[Computer Configuration/Administrative Templates/Windows Components/Remote Desktop Services/Remote Desktop Session Host/Connections]
Windows XP/2003
[Computer Configuration/Administrative Templates/Windows Components/Terminal Services]
Optionally, enable "Restrict Remote Desktop Services users to a single Remote Desktop Services session"
in [Configuration\Administrative Templates\Windows Components]....same paths in Local Group Policy Editor as above.
Reboot.
Create an user account for each person who is going to connect, with passwords, permissions and everything;
Make sure you have the ability to have multiple RDP sessions to the same Windows machine by actual testing. If not, find a solution (like buy a license, or google for how to raise the limit of maximum active RDP connections).
Now every time you want to do the desktop sharing thing:
Have them log in through RDP (the target session has to be a RDP session too).
Via [Task Manager/Users] each person in a RDP session is going to have to right click on the target (common) RDP session and click "Remote Control".
Note: localhost RDP connections can be made using localhost IP's: 127.0.0.2, 127.0.0.3, etc.
Note2: You can enable "Set rules for remote control of Remote Desktop Services user sessions" for an individual account, for security reasons, by altering this setting while logged-on on the target account, and instead of going to "Computer Configuration" root, start from "User Configuration".
Best Answer
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.