Dear @awelzel. The scenario you described should work: adding users to a group that is part of a collective should make the collective available to these users immediately. I use this setup myself and it works well.
Could it be that you face this issue on an instance that got installed before Nextcloud 22? In this case, it might be that the migration of circles to Nextcloud 22 didn’t run successfully. I faced this in the past. Running occ circles:memberships --all solved this issue for me.