You can login several user at a time by enabling "Remote Login", however this will enable many user to login via ssh / scp means. Which might be workable if you want access to terminal tools including xcode's command line interface for building projects you can use things like gcc or xcodebuild via a ssh session.
If getting each additional user access to their own GUI is your goal, I am not sure that is possible with the built in VNC server. There is an interesting solution
10.4: Use multiple simulataneous GUI accounts noted on Mac OS X Hints that may be the answer to your question, however it was tested on Mac OS 10.4, not sure if it sill works on OS X Lion, etc.
Their solution is:
After you have two or more accounts set up on one Mac, install OSXvnc and start the server. Once started, Fast User Switch to the other user on the machine.
Now from another computer, use a VNC viewer like Chicken of the VNC and point it to the Mac. Even though the Mac is logged in as another user, your VNC session goes to the original account, allowing both of you to use the machine at the same time.
When testing, this is seemed to work well, although you have to wonder if there will be problems with system calls from the same programs happening simutaneously. I've only been running this for about two hours, but no problems so far.