Clans:
Clans automatically join Battles when the leader reaches profile level 50.
New clans can join Clan Battles every Monday or Thursday, provided that the clan leader's level was 50 or higher 1 hour before the Preparation Phase.

If a clan joins, it can no longer be removed from the list of participants.

Players:
You will be able to join Clan Battles when your clan leader reaches level 50. 
After joining a clan, there is a 24-hour waiting period before you can take part in Clan Battles.