I think your almost there actually...

Once you have created a new "Group" then you must create a Forum Topic.. Until a Forum Topic is created by the Admin then no Members even with privilleges will be allowed to add NEW topics. But once a topic is created the Members with proper "Actions" can participate openly.

The Admin protected in the Forum is deligated to the "GROUPS CREATOR" in the Orca Groups Feature of DOLPHIN. So if a Member with proper "Actions" see more creates a Group with no Forum Topic then no joined or invited member will be able to post.

The folder icon directly across any Group, Forum is a creator button.

Let me know :)