Hello Peter,
The main purpose behind the Assign roles button was for instructors to assign different presenters and hosts for the meetings.
Any instructor that creates a meeting is given the "host" role automatically.
if a user, who has no adobe connect role clicks on the join meeting button they are given a participant role by default. Also public meetings be design people to join the meeting as a guest. Where as a meeting created as a private meeting, will only allow users who have an Adobe connect role for the meeting.
Regarding the group mode, separate groups will create 3 separate meetings and only allow users, who belong to that group, to join the meeting that pertains to the group. Here is some additional information aboug groups mode. Here is some information about the module
Hope that helps.