Coronavirus: any open source WebRTC conferencing website without login?

Check out https://meet.jit.si/ - no account needed. I believe all rooms are public, but not discoverable.

I don't know how well it holds up for large groups, but for ~5 or so clients it has worked nicely for me.

WebRTC experience will be highly dependent on the network connection of the participants and can only handle so many connections. If you turn off video, it will handle more people and lower connection bandwidth.