Another person was assigned to help me on a project so I had them install Flash Builder 4 and told them where my workspace it. Now I get an error when I try to open the same workspace when they have it open. How can I have multiple people working on the same project in Flash Builder?
Yeah just sharing the workspace files is not going to do it for you and create problems as you have noticed. You'll probably be best to look at using some kind or source control like SVN using subclipse maybe with Flash Builder. There are many solutions out there but that is perhaps one of the most popular.
you can find subclipse at http://subclipse.tigris.org/ that might give you some ideas on where to start moving forward on team collaboration with Flash Builder.
Hope this helps,