User permissions and groups

Hello all,

I'm fairly new to Drupal and am trying to work through a permissions system. I have the need for about 30 or so groups so I initially went with Organic Groups. The problem with this module is that if a user in group A creates content for public viewing, then everyone can edit that content which is not desirable.

I would like a way to easily create a group, assign members to that group in such a way that content created by someone in that group marked as public is only editable by someone else in that group but not by the general public.

What is the best way to proceed?