Multisite advanced, Groups

I want to install a multi site with a subdomain(dru 5.7).
This is what I want shared, how do I do this?

I want all users shared, and sessions, but I also want groups shared.
IE the sub domain will be the main groups area where they will have their own pages but I still want the groups to be open and changeable on main site(Group owners can subscribe and and manage members). This way users can subscribe from both areas and it shows the groups on both areas but I only want the group owners to be able to create their pages(ect) on the sub domain.

Also is there a place I can look up table prefixes matched up with what they do?
* 'default' => 'main_',
* 'users' => 'shared_',
* 'sessions' => 'shared_',
* 'role' => 'shared_',
* 'authmap' => 'shared_',
* 'sequences' => 'shared_',
These are the only ones listed in the setup file. Where do I get more from? I know I can just look at the db but I wanted to know if there is a place that describes the data each one holds.

Thanks in advance.