Thanks for your encouragement. I’m glad that my experience is useful to help your work through your own journey.
I must say before I reply that my speciality is not online community, but offline community. I have always used online as a way to augment offline (indeed, online and offline are just ends of a spectrum that has little divide, really).
If you stop thinking about forum groups and think instead about relationships, your “sub group” of hypertrophic cardiomyopathy patients already exists. They have a set of relationships that has formed a community, existing across various channels, private and public, regardless of whether there is a formal sub group for them.
Creating one would of course help them - however - if they don’t have the motivation to create it or contribute to it, then you could end up making it, to only find it is a ghost town.
I would go down the route of empowering these early adopters, and asking them what would help them, and indeed giving them privileges that enable them to do this a bit.
The best way to do this, in my opinion, is to draw that group of people together. Have a hangout or group Skype with them, or an email chain, or an offline meeting, and listen to the conversation.
Then those who are interested can be empowered with more privileges. The pitfall here is that some people might resent not being given privileges, so it might be good to limit the number, and allow people to nominate themselves or something like that. This way it is seen as a community appointment, and something that you are testing as you roll out.
Another pitfall would be that you create a subgroup for them, but there is no activity there, because they just don’t need the formal group. This is why listening to them is critical. It might be that they don’t want a subgroup, but would really value a monthly Skype call with you, for instance.
Is that helpful?