Fan Clubs
From LeakyPedia
Mischief Managed!- Chicago Marauders
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
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.