Your clan's command post: manage the roster, send and answer challenges, follow your Clan Battles and change your clan's settings, all in one place.
They see the invite on their own Clan HQ and accept or decline it there.
One active battle per clan. Both clans need the roster for the size: 3 for 4v4, 6 for 8v8, 12 for 16v16. The exact time is agreed once they accept.