Even if you have the profile, you won't be able to use SharePlay with the iOS 15 RC.

Yes; their actions are coordinated. If user A and B are in a SharePlay session and user A seeks forward user B will see their video seek forward as well. Your player UI should already be monitoring for changes in player state (play/pause, playback position, buffering, etc.) and update the UI accordingly. But even if you don't, you should still see one user affecting the other users player.