How to give a group by default to a new contact?
One option is to do it with profiles. You can automatically add contacts to a specific group when they are entered via profile. Create two different profiles for your two groups, and have your users enter all contacts through their own profile page in Profile-Create mode. However, if you find yourself needing to create a unique profile for every user, then it's time to re-assess the need. An extension would be better at that point.