The course-adjustment is set to seperate groups.
Then I create a group-forum that is also set to seperate groups. But when a new student enrolls to the course who is not in a group yet, he is able to enter the forum via the "recent activity" window. He can read and write posts. When he wrote an answer to a post of a certain group, the actual members of the group are no more able to write an answer.

I am not sure if this problem is forum specific or if it relates to other activities aswell.
Since we are currently implementing a new activity for exercises that have to be solved per group, this could become a problem to us. Because we don't want a group reading the exercises or solutions of another group.