You get a bonus for inviting your friends and then they have to invite their friends.
"New players can enter your recruitment code, which will earn you rewards. A certain number of players must be recruited in order to earn exclusive dragons."
"Minor in-game currency bonus for friends who join or click on the player's link"
"It requires 10 persons to join and play to a certain level for little to no rewards, just one dress set."
"In the fortresses, the higher level chambers can only be completed by playing with friends."