The way it works right now is (as of 4.0 integration):

• In a course, the mode can be no groups, separated or visible groups.
• If there are no groups, would only create one BBB room for everybody enrolled in the course
• Visible groups:
  • Will create (on join) a separate BBB room per group in the course
  • If user is not enrolled in any group, user is allowed to use "all-groups/0" room, user is sent there by default
  • If user is enrolled in only one group, user is allowed to use "all-groups/0" room and their own group room, They are shown with a drop-down menu to chose between them
  • if user is enrolled in multiple groups, the user is allowed to join any of the rooms enrolled or the "all-group/0" BBB room. Users can choose from the drop-down menu
• Admin and teacher always have access to all the groups in the course, therefore they can always chose, therefore drop-down is always there
• Separate groups would do the same as Visible group excepted that it will force student with no group to join the "All participant" group by default.

The recordings visibility follows the same logic:

• If student in a group, it will show recordings from this group and the group "All participants"
• In separate group : Users with no group will be able to only see recording recorded for the "All participants" room 
• In visible group: 
  • For Admin it will show all recordings
  • For other it will depends on the currently selected group

There is a discussion to have ref. the way it works in other modules. From an earlier comment from dobedobedoh , it seems that forum would not allow anything to be done with users (except maybe read)  which are not assigned to a group.

The way groups work in other modules:

• No groups - There are no sub groups, everyone is part of one big community
• Separate groups - Each group can only see their own group, others are invisible.
• Visible groups - Each group works in their own group, but can also see other groups. (The other groups' work is read-only.)

Translated into BBB, it would mean that:

• No groups:
  • Joins: user who can usually join (moderator/student permission) can see the join button.
  • Recordings: users would be able to see any recordings (even recordings from subgroups if ever the course has used groups before and is using no group currently).users not assigned to any group would not be able to join or see any recordings.
• Separate groups: 
  • Joins: we can only join the group we are part of. If we do not belong to any group, we cannot join the meeting (except maybe moderators ??)
  • Recordings: same logic: we can only see recordings from this groupe
• Visible groups:
  • Joins: we can choose which group to join in the meeting (but there is no more possibility to do it as a user that does not belong to any group  / group 0 ??, so user without a group cannot join anymore)
  • Recordings: can see recording from its group or other group

For the forum in visible groups:

For the forum in separate groups: