Make it clear to non-prime subscribers that they need prime to participate
One of the complaints that I received during one of my recent watch party streams is that a viewers messages were never appearing in chat. This was because they did not have a valid prime subscription. My recommendation would be to treat the watch party like a subscriber only stream and/or like a VOD for the duration of the watch party and lift those restrictions when the watch party isn't happening. It would also be nice if there was something to tell them to connect their prime account or direct them on how to get prime so that they can participate.
We appreciate the feedback! Our team is currently working on various designs to make it easier to onboard new Prime subscribers. Keep an eye out over the next few weeks for those new user flows!