A friend of mine just described the following problem:
They scheduled a Jitsi meeting for 3 people. When they all entered the conference room via the very same link at the appointed time, each participant was alone and all three had the same user name. Two of them were using the Jisi Meet App, one was using a browser.
After some phone calls, they managed to start the meeting by reloading it in the App. However, this was quite a disturbing experience for them. Since we are planing a bigger meeting with Jitsi Meet tomorrow, my friend is really concerned about this issue.
Another friend of mine also just reported a similar incident. In her case two people were in one room and one found herself alone in a separate room (all three used browsers). They solved it by reloading the conference link.
From similar reports in the Community Forum (see below) I can guess, that the issues, I just described, might result from firewall problems.