In separate group mode, users who can access all groups (teachers by default) can never join meetings with group bound users (students by default) even if they are members of the same group.

Steps to reproduce:
1. Create a course in separate group mode. (separate group mode is not strictly necessary)
2. Enroll a student and teacher into a single group in the above course.
3. Create a BigBlueButton activity in the course, making sure it is set to separate groups.
4. Click the activity as a teacher.
5. In another browser or on another computer click the activity as a student.
You will see they are now in separate meetings.

Inspection shows the meeting ids are different. The teacher's meeting id ends in 0, whereas the student's meeting id ends in their group id.

The function groups_get_activity_group returns 0 when the user in question can access all groups. (teachers by default can do this)

A solution would be to intercept 0 and multiple groups and allow an interstitial screen which requires the selection of a group chat to join. The meeting ID would use this ID as the last member in the meeting ID chain.