I am not aware of being able to join online. Have you found out why your campus doesn't have any? This may determine your aibility to join/start one on your campus. Where are you going to school?

If your campus doesn't allow GLOs maybe see if there are alumna groups in your area that you could get involved with and join as an Alumna member. I know AGD does have an Alumna member initiates, so maybe some of the other groups do as well.