There is a PAF utility program called PAFInsight that allows you to compare and synch two PAF databases. You can then import the missing people. It costs about $25 from http://www.ohanasoftware.com/http://www.ohanasoftware.com/ It is the only product I know that can do this. (No gedcoms are needed for the process, as it works with the PAF databases directly.)
Or if you can afford to be patient, the LDS church is beta testing a website that will allow you to create a collaborative family tree online. I haven't seen it yet, but the basic description indicates that it may be what you are looking for. i.e. Different family members can be building the same family tree. Ask at your local Family History Center whether anyone there has worked with it as a beta-tester. It is expected to be in operation by the end of 2007.