Mandatory member of an organic group upon user profile decisions

Hi,

I have this problem: when an user creates his profile (upon registration), he has to make some required decisions regarding that profile. E.g. he has to choose the city he lives in, which is a part ot a taxonomy tree (therefore content taxonomy is used). This taxonomy tree looks like this:

e.g.
Europe
- France
-- Paris

I want this user upon choosing Paris as the city he lives in to be then automatically a member of the OG "Paris", which has been created by me (admin) before that. He won't have to make any more choices...

How could you achieve that?

Any help is greatly appreciated! Thanks