When you build the team meeting hall, you will be able to join a clan. A clan is basically a group that can help you defend your city and help you fight other people. The Team Meeting Hall is near the Dragon Stables and the Breeding Den (it's in ruins until you reconstruct it)
Every 13-24 hours you will get a certain amount of egg tokens, depending on your league's level, and if you have won any wars (depends on how much your clan takes from the other's treasury).
Joining or Defending Clan Members
If you are in a clan, you might see a button pop up at the top of your screen saying "Join Attack" or "Defend Teammate" or something like that. If you want to have help for attacking someone after you choose the dragon, while you are waiting for the battle to begin, there is a button beneath the countdown that says "Invite Teammates".