If you want to do it lots of times going forward you want the API, if it’s part of a migration then you could use the Rails console or an import script.
If it’s something that you’ll do forever and there’s some trigger that would add them to the group, then you might use a plugin.
Oh, look! I wrote a GitHub - pfaffman/discourse-user-creator: Create an activated user, optionally assigning to group · GitHub. I don’t know if it works, but it probably does. You could conceivably modify it to just add to groups. If you would like help modifying it and have a budget, my contact info is in my profile.