Darned good question! I'd like to know this as well.
I also GREATLY dislike the fact that when a server admin adds a new user while in a Group that the user is not added to the Group but just added to the general population Not logical at all!
I just added over 1600 users via csv file import. It took me less than 10 minutes. We use SAML and AD. I created the file with just user ids to import. Once imported they came in as unlicensed. I selected all and modified their group membership then I modified their site role from unlicensed to interactor. Piece of cake.