Since these are user settable custom fields, you can just make the groups open membership and tell people to add / remove themselves via the /groups page.
Not realistic for my use case. Hard to get a bunch of random homeowners, some very old, to bother going around tinkering with their settings. It was hard enough getting them to just register once to get the emails, and it’s hard to even get them to pay attention and do stuff even when there are financial penalties.
For anybody coming across this in the future: I had a similar use case (two distinct user populations – people offering services vs. people seeking services) and ended up implementing it via webhooks and the Discourse API.
More specifically, I…
created a webhook triggered on user events
implemented a barebones Express.js server to receive the payload
set up a POST route for user_created events that checks the value of a custom user field, and then uses the Discourse API to set the user’s group accordingly
This seems like kind of a goofy way to do it because I’m going outside Discourse to modify something within Discourse, but I scoured the forum for answers to this and came up with nothing, so I did my best! Hope this helps someone in the future, and if anyone comes up with a better way of doing it, I’d love to know.