From LeakyPedia
The Three Broomsticks - Penn State University
Mischief Managed!- Chicago Marauders
OotP - Saint Andrew Junior High, Nova Scotia, Canada
Atlanta HP an adult Harry Potter group in Atlanta.
The Group That Shall Not Be Named - Harry Potter NYC
S.L.A.W. - St. Louis Area Wizards
The Seattle Harry Potter Meetup Group
Los Angeles Dumbledore's Army Meetup Group
Veneficium: Acadia University, Nova Scotia, Canada
Rowling House: Greater Phoenix Chapter Bold text
Don't know if an online group counts, but I'm an administrator on the Babycenter Harry Potter board.
The Poll Castle Commentators! We meet at the comments section of the Poll at TLC. We are about fifty people in number who have been chatting for over six years now.
Harry Potter book club disscusion group in IL
FelixFelicis - Spanish Harry Potter Group
Harry Potter Club- Fieldston Middle School, NYC
Harry Potter Club- The Computer School, NYC
Defense Against Dumb 'A's (who don't get HP) - The Northern VA HP Meetup Group