It seems to be a popular service generally supported by browser add-on or other hacky methods; but wondered if it would be possible to support natively?
I could see iCloud being a possibility - though unsure how latency stable iCloud updates are for this kind of thing.
I realise the only reliable way of doing this may be you guys running a webservice, which would eat into funds, perhaps additional tier and/or perk for lifetime members?
Alternatively, self-hosting could be another way to do it.
Anyway, in short, multiple clients able to play the same content in-sync (1s or less); with perhaps the ability for one/many to control play/pause.