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.

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