Beside the AStA, there are several other groups that fight for students in which you can also become active, like the autonomous departments. As the name suggests, they are autonomous and independend from the AStA. They each stand up for different minority groups at university and are open for counseling and participation from students.
Here’s an overview of all autonomous departments of the AStA:
Fem*Ref (feminist committee)
International Student Group (ISR)
Committee for students with handicaps and other strenghts(RSB)
autonomous committee for gay students (Schwur)