There's been a lot of users asking this question in main (even though no one bothered asking here) so, I'm going to answer it here to avoid the toils of having to explain it in PM and what not and possibly let users who may come here to ask this know what to do.
"Can we have our groupchat become an official chatroom? it has a lot of users!!! Oh, official chatrooms are no longer being accepted? Surely we can get a private room!!"
Well, if you have a Group-chat that you think is popular enough to get a privateroom, you should get that notion out of your head for now. Group-chats have existed for a little more than a day and is still being hyped up, so a lot of users will be participating in them now and if you think this is the actual userbase of said room, you're wrong.
As time passes, the userbase will either grow or decay and if you make a post asking for it now, will get rejected because you can't know for sure if this is the actual userbase. So, wait 2 weeks, see if your groupchat's userbase meets the quota for a Private Room and then post.
In the meantime, you shouldn't be advertising the groupchat in any chatrooms or spamming random users' PMs to get your userbase up. You should only invite people you know as they'll be the ones sticking around for longer periods of time and will give you a real answer as to what the userbase might be.