Mastodon groups is not the same as Guppe groups, they work in different ways.
And, even if they were the same it is really dangerous to hand over all the Fediverses features to the same development team. The point of the Fediverse is to spread out across many servers and many kinds of software, so that no one has power over it all.
Also many servers don't run Mastodon software. Guppe is server agnostic, whereas Mastodon Groups seems to require a Mastodon server.