It says ura_limit_reached. I'm guessing you don't have enough paid accounts.
Edit: it appears there is a 5 guest limit per paid account.
Edit 2: I think I'm going to set up a Mattermost server and push people that way.
Rooms are of an unlimited size and have all the core functionality like file & image sharing, code highlighting, "reactions", which we've come to expect but also focuses more on getting people to speak to each other. It's built on electron and is both really quick & also require almost no resources when running in the background.
Edit: Note this statement is based on my talking to people actually at matrix, on the dev team.
It may be aiming for it, and I do hope we see an open chat protocol succeed over all the Slacks and HipChats, but calling it the successor right now seems a little disingenuous!
For non-tech people, yeah, it's probably easier.