Sounds like you're connecting to a Windows XP Pro machine
By adding users to the 'Remote Desktop Users' group, or granting them 'Allow logon by Terminal Services' rights in - it will enable remote login. Sounds like you're seeing a slightly different side effect here though. Windows XP Pro is only licenced for one 'active' local or remote session. RDC has two roles, a) allow remote-administration e.g. by a domain administrator and b) mobile worker to access his desktop from home.
Sounds like the problem starts because someone is already logged-on when a remote user tries to connect. Because its a strict one-out, one-in policy, that interactive user has to be logged-off.
The only people who can log other users off are people who are members of the Administrators group, or the user themselves. Your remote users will need to possess Administrative rights if any other user is left logged on.
There are somethings you can do to help aleviate this - e.g. setting up the auto-logout screen saver from Win2K sdk (google for winexit.scr) However even this isn't foolproof.
The constraints on Remote Desktop are documented in the support system and on the MS website.
http://www.microsoft.com/windowsxp/remotedesktop/faq.asp
If you need more parallel sessions you'll need to go for an MS or 3rd party terminal services system. VNC is reasonably good - it's client is a little lacking compared to MSTSC though on features like sound, clipboard, printer and drive mapping.