When you join an alliance after the preparation phase started, you’ll have to watch the battlefield in spectator mode. It lets you see the teams, who’s attacking who, but you can’t attack.
So… idea: we should be able to watch in spectator mode the battles of all alliances. There’s little private info that could be gathered: we can already see when anyone is online and their raid defense team. Also, up to 60 players get to watch the battlefield, a few more watching doesn’t change anything.
So this would be more for studying battle strategies and cheering your favorite team.