The creation of additional accounts will be improved in the next release. You won't have to create a regular sub-account and then go into the settings to change it to a group, it will be created a group account and ready to use as one. I submitted another change to do the same for creating an Org or News page (but that hasn't been accepted yet).
So far as I can tell the main difference between a "Restricted" and "Private" group is the default visibility of posts in the group. Posts in a Restricted Group are still public. The restriction is on joining. Posts in a Private Group are supposed to only be visible to members of the group. But I gather it's labeled "Experimental" because that doesn't always work as intended? Of course a Private Group can still purposely post something publicly too.