How can I customize the registration form on a template group basis?

I need to customize the registration form for several template groups, i.e. each template group has its own registration form. The reason for this is that each one of these template groups refers to a distinct “project”, so each newly registered member must be assigned to his or her project.

As the number of participants can become quite large, I need to figure out a way automatically to assign newly registered users to their projects.

Each project is identified through a channel entry, and each entry has a unique project ID in the form of a string “prj_1”, “prj_2” etc.

I tought I could add an hidden custom field to the registration form. This custom field would then be filled out with the appropriate project ID. This can easily be done. What I can’t figure out is how to pass the project ID to the registration form.

Does anyone have a suggestion? I would really appreciate it!

-Franco