I know a lot a people with ADHD (including myself) and most of them do not participate in ADHD communities because the disorder is not a big part of how they identify in life. They just accept and deal with their ADHD as best they can and get on with things. I have ventured in /r/adhd from time to time and even commented on posts, but mostly I don't feel like the community has anything to offer me. I don't need any support and I've accepted who I am, so I participate in other communities that actually interest me.
I was going to create a place like that but then I started learning to play guitar : ) Edit - Jokes. Sorry. I agree with you. I don't really like hanging out in groups where people go to vent or constantly rant about negative things.