Automatically add new registrations to public usergroups

[cocjh1]

I want to automatically add new registrations to all pubically open usergroups on my board, which they can then opt out of if they so wish via group memberships.

Found the hook where to enable this (register_addmember_process) and the table 'usergroups' lists all the usergroups.

Question is, which table records relationships between users (userid) and 'usergroupid'?

Think I need something like:

FOR each usergroup
   IF usergroup.ispublicgroup
      ADD this user to permissions table

But no idea which table to effect the permissions change on... :erm:

Any ideas??