New setting for subgroups that allows members from main group to join without admin approval

[despens]

Currently, subgroups have two access settings:

• Closed means that members of the main group can see the subgroup and its contents and ask to join
• Secret means that the subgroup is invite only

For certain kinds of volunteer work it would be nice if there was a third setting, maybe "open", that allows members of the main group to enter (and leave) a subgroup without admin approval, skipping the whole step of requesting and approving subgroup membership.