For User creation, use the out of the box behaviour of the Discord login.
For group membership you are looking at something more complex.
@spirobel has a plugin that syncs Discord roles → Discourse groups, but he charges for it (good for him), so maybe discuss with him privately?
If you want to contribute to the main open source Discord/Discourse bot project, look at:
(but bear in mind it currently does not have fully automated sync (it is command initiated) and it in any case syncs the other way, but would give you a foundation in any case.).
There is also this plugin, but it doesn’t seem to be actively supported, and again goes the wrong direction. It is based on my code: Discord Sync: sync a Discourse forum with a Discord server. There is also a useful discussion about the topic further down.