Jitsi Meet. They can just paste a url into their browser and that's it.
The URL can be an ordinary long string or random sequence like
http://meet.jit.si/somerandomplaceontheinternettomeetme
Remember though: meet.jit.si, like other WebRTC video conferencing tools is nothing but a coordination point/SIP, a rendezvous server that then connects you peer-to-peer via the protocol for 1-2-1 calls or a Videobridge for groups. Unlike Teams and Zoom though, you can set up your own, which is a significant security and performance value-add.
When Jitsi (WebRTC) struggles it seems to do a sensible thing and fall back to preserving good audio but temporarily dropping the video completely.
One or one calls should be using direct peer to peer connection thanks to webrtc.
Group calls will be routed by the server, so latency will be an issue depending on the server location.