You can press the
NEW button under the
Custom and under the
Automatic tab (in group settings). They each have their own url, but it’s actually the same form.
If you add a new group from the automatic tab, the new group will even show in the automatic list, only to be moved to the custom list after a refresh.
This is confusing. For a while I thought I could make automatic groups (which still is a cool idea I think). An automatic group would be like a badge, only then people will join that group as well.