If for some reason somebody joins late / gets kicked off, and you're watching something on youtube, youtube supports time-index URL fragments like this https://www.youtube.com/watch?v=F4-SxcCO5d0#t=3m17s , and you could start a new watchparty with that URL.
It's heavyweight and probably better to just sync by somebody shouting a timestamp, but if you need things synced there you go.
Just calculating from the start time at what point in the movie everyone should be. e.g. You are half an hour late, so the others are around minute 30 (Y)
It's heavyweight and probably better to just sync by somebody shouting a timestamp, but if you need things synced there you go.