there are many forums about Harry Potter
Here they are!
http://radnet.org.uk/ (it is the forum of Dan Radcliffe, I hope you don't mind that it is mainly for Dan but not Harry)
http://www.cosforums.com/ (mugglenet forum)
http://www.veritaserum.com/forums/ (veritaserum which is very famous in Harry Potter world. it is a very excellent website.)