BuddyPress.org

I am currently developing a site where the subject matter is specific to geographical regions and cities. My initial thought is that I would set up WordPress multisite and have each region / city be a separate subdomain.

I would like to allow users to join a specific city or group of cities. If they join a group of cities I would like their profile information and activity to carry to others.

I was wondering how BuddyPress handles that if I install it on the root domain?

For example I would install BuddyPress on http://www.example.com. If a user logs in and goes to newyork.example.com will all of the BuddyPress information be available as well?