I don't know if I have the time/bandwidth to join a group, but it sounds like a super fun idea that I would love to participate in (if only I had the energy to do everything!)
Private will most likely not be possible, at least until WAY later down the line - renshuu is a small enough company that we (I, really) cannot manage the potential reports for content that come from private communications, which is why the private DMs were removed earlier this year.
I'd also be interested in this! There are language exchange apps, but I think that learning alongside other learners would be a really unique strength to renshuu. I don't personally use Discord, so there's not really anywhere else to find this sort of thing for me.
Something I'd like to see personally with this new feature (and even outside of this feature like on the Friends list), is an option for a team-based learning where everyone's contributions can affect. For example, a shared level bar for the group or other rewards, or some kind of system where you all can see each other's progress and, if you see someone struggling/lagging behind the group with a specific word, maybe the others in the group can see that and all pitch in to create mnemonics for them or discuss it. Perhaps something kind of like "baton passes", where if person 1 studies, and then person 2 studies, it increases the chain and awards person 2 at double XP, and then person 1 can study and recieve three times the XP, that way it motivates you to take advantage of someone else studying and make a chain reaction.
Anyway sorry for being long-winded, it's just some random rough ideas, hopefully something in there looks interesting and catches your eye or even makes you think of something totally different, and if not, that's ok too! Thanks for all your hard work, マイコー! renshuu truly is a masterpiece and has helped me so much!