Ability to let group creators assign default membership at group creation

There's a way for the site admin to set up a default membership level that new group signups will automatically be assigned to when they join a group.

There's a way for group admins to change that assignment for a user, one at a time, for people who are group members.

What would be _really_ super-nice is if, when a group is first created, the group owner could select a different role as the default member role for new user signups.

For instance, here's my case:

I have some roles:
group member - can view group posts
group contributor - can both view posts and create posts
group admin - can view, create, and edit posts
group owner - can view/create/edit posts as well as change settings on the group itself.

Currently, my default setup requires that there be group roles where a person is not necessarily allowed to add posts to a group, but can still view them, but some group creators may want to allow their new signups to be able to post also, on a per-group basis, while others will want their group's content to be read-only, and invisible to non-members.

Currently, the user roles setup makes new group members have the most limited access (view) and if a group owner wants to allow group members to also _post_ to the group, they have to manually go to the subscription page for the group and assign the edit access one at a time.

It would be _fantastic_ if a group owner could set, at create and edit time for a group, whether new users would get the group member OR group contributor status.

Is there a way to do this already, that I'm just not understanding? Or would this be a new feature request?