Well, I see what you’re saying, but if you need to put users into groups very rarely, then writing code to use the API is hardly any more efficient.
Consider if you have say 200 users and you want to assign 190 of them to a group. You could then easily select all users with the “select all” feature, then uncheck the 10 that you don’t want, and hit “assign to group” or something.
But obviously this doesn’t work when you cannot select multiple users and assign them to groups.
Allowing people to join is unfortunately not the same as putting them into the group (e.g. for my use case, which is assigning them their own category which only this group has access to).
I guess the conclusion is that you cannot easily assign many users to a group. Not without writing code or using the API some other way.