Provide option to subscribe all users to all groups

Issue Summary

Thank you very much for this module. It's just what I need.

I've got a couple of sites that are intranets and all the groups are available to all users. Right now, I have a three step process:

1) Create group
2) Add group to "available groups" list
3) Assign a role all users are in to the new group

What I'd like to fold steps 2 and 3 into step 1 by adding the following fields on node/add/group:
a) A checkbox for each role with the following description "Add the users from following the roles to this group automatically"
b) An option to apply this setting retroactively or just to new users

I'd be willing to help cover the costs of this feature. Please let me know.