The reason that’s not working is because when Discourse tries to send a Follow to a.gup.pe/u/koirat/inbox it’s getting a 500 internal server error in response.
HTTP/1.1 500 Internal Server Error\r\n
In other words, there’s a bug in Guppe.
I would note that the collective fediverse approach to groups is currently evolving somewhat. I’m discussing it, and a few other related issues, currently with folks from NodeBB, Wordpress, Mastodon and others. See for example:
The other issue that Guppe is going to face is that Mastodon itself has been working on a “Group” feature for some time which may conflict with the Guppe approach.