I started working on private groups in Mitra. Beginning with version 5.10.0, you can follow private communities on Lemmy 1.0 beta (test instance: https://voyager.lemmy.ml/), post to them and leave comments on posts.
Local private groups are not supported yet, but I hope to finish this part soon.
I will also try to make Mitra at least partially compatible with Shoot - an instant messenger built on ActivityPub that has private groupchats based on FEP-1b12.
RE: https://mitra.social/objects/01a03596-55ae-7562-96a7-830312413610
Quote pending approval
