@pabloscloud I can assure you that back in the day before Mastodon when we had groups, there were still different servers.
We even had groups intended for the users of one server, to strengthen the ties of that community, as there was no Local Timeline on GNU Social, and there still isn't to this day.
To correct myself: There have been groups all along, ones that Mastodon can use too, you just couldn't create them in Mastodon itself.