I am working on a site where self enrollment shall be possible. 

We have several groups. The size of the group varies, so we don't know yet how many persons enroll for each group. Each group will be allowed to enroll into all courses once a small payment has been made for the group.

How can we best do that without the hassle of issuing  passwords for each course?

My idea was to issue a password which would then assign a person automatically to a global group. Any ideas?