In Windows XP on a Domain joined system only a single desktop is allowed.
Microsoft’s Remote Desktop or Remote Assistance only allows a single person to remote into Windows XP at a time. Remote Desktop will lock out the console allowing only the person using RDP to access the system. Remote Assistance is a peek over the shoulder to see what the user is doing or share the desktop at the user’s rights level.
In order to have multiple people share the single desktop simultaneous for administering or support a system you can use a third party remote assistance product.
If you are looking for a creative method of having multiple people share a single desktop for administration or support look at netmeeting or other desktop sharing for meetings applications.