This may be related to an issue I noticed today, when working with multiple rooms (but fewer participants, only 1-2 per room). Some individuals would not see a popup, which was dismissed as browser finicky-ness, and they still received an invitation to the room that appeared on the user bar to the left. However, some did not get an invitation at all. Also, some rooms would load without the background slide.
This happened today in 2.4.-beta3.