Start offline - establish a location, time/schedule, snacks, etc. Advertise online (e.g. craigslist, local forums, via social media such as with school, city, library groups), put up flyers in advance (e.g. weekly starting MM/DD/YY. Try to find a group of core members because someone will need to keep track of things
Combining: bans, blocks, defederation, differing lemmy versions and federation bugs / glitches, you end up with a different view (sometimes for better or for worse) than someone on another instance