I'm guessing the answer is you can't do this but is there any media player or anything that can play videos so that everyone sees them at the same time?
Hi! The answer is yes.... kinda...
The standard media streams (MOAP, etc) will do exactly this... so long as everyone is present and has the viewer setup to automatically play media from prims AND noone touches anything lol... A good free and opensource alternative to viewer-based prim media would be,.... https://togetherjs.com
It allows mutliple anonymous users to connect to any page and provides the said group with collaborative browsing/viewing videos/etc.
Let me know if you need any assistance, I have quite a bit of experience setting up this platform for such uses.