Forums (fora?). These communities are where the real work of rocket mass heater evolution happens. Asking questions, making introductions, finding each other in local regions, sharing results of explorations. The first two listed here were the first developed, and the best. The Facebook groups don’t get terribly detailed, but are a good way to locate others, find workshops, and share images. If you know of other online forums, holler!
Donkey’s Bulletin Board The first, started by Kirk “Donkey” Mobert, several moderators, a passionate kind scene.
Permies dot com, a hang-out for lots of permaculture topics. Moderated by Ernie and Erica Wisner.
I can’t speak to the quality of this one in the UK but it seems to have activity:http://woodstoves.forumotion.co.uk/forum