this really isn't a supported feature, so there isn't much beyond a general overview I can offer.
your strategy of adding the UserGroupID to the contents table is the correct way to go.
you would also need to add a way for the supper admin to assign the content areas to the user groups, a drop down list on the contents page should be all that is needed for this, then update the insert and update behaviors to use the usergroupID selected from the list.