About 35 persons gathered to listen to a presentation on VR. At the end of it I gave them Cardboards and the URL of the demo http://cafe.vrlab-brussels.info/ . They connected using their own phone to a first scene to explain key concepts of VR. Then after a bit of playing around they unlocked a link (watch the moving ball behind you then look at the yellow cube) to move on to http://cafe.vrlab-brussels.info/multi/ and were able there to navigate in the same virtual space (pressing the Cardboard button to move forward, turning the head to turn).
At least 15 managed to connect simultaneously. At least 2 of them managed to meet each other in a specific spot (next to the "statue"), recognize each other using a uniquely coloured "head" (no mirror present, has to be collaborative) and race to another element of the scene (a tree).
Nobody installed any app or had VR hardware specific device and for half of the crowd it was their very first VR experience.
It's done using Aframe with the firebase and universal-controls components. All the code (sic, 30 lines top) is there for you to see and tinker with.