I don't know if it's possible to create subrooms or it's created automatically... The owner of the chat should create their own subrooms...
Subrooms are created automatically at the moment based on the number of users in the chat (50 by default, you can change it from 10-200). e.g. If you have 105 users, you would have 3 subrooms.
Owners creating custom subrooms is a good suggestion and is something we have on our list of features to add. (in fact we just bumped it higher in the list since you asked)
In the meantime you could create a different chat for each subroom that you want.
We have our roadmap listed here: https://wiki.minnit.chat/Roadmap
This feature isn't on the roadmap right now, but it is planned.
We still have to evaluate other features we plan to add in April, and this is likely going to be one of them.
If more people ask for it we'll consider doing it sooner.
I think that feature is important to dintinguish this chat application from others (I searched and I didn't find it).
It's listed on our roadmap now: https://wiki.minnit.chat/Roadmap
I can't give you an exact timeline on when we'll be able to work on it as we have other features we're working on currently.
We appreciate the interest and this feature is one of our top priorities right now, however it's hard to say when we'll have it in place.
The best estimate I can give is that we'll have it released in late May, or June at the latest.
I can't give any promises on that timeline as we have bug fixes and other features to implement.
I'll make another post when we begin working on custom subrooms.