The idea here is that you have general categories and then you rely on tags to do more granular filtering. so you may be on the gaming group, but if you really hate retro gamess you would instead add games.retro (or something similar) to a filter list.
The idea of only a few groups and no custom group creation is intentional. There were other reddit alternatives that died out because everone was creating new groups willy nilly and it meant no one group could get enough traction. In contrast, Tildes only started with a dozen groups and you start out subscribed to everything.