You may need a helper or several helpers for some crafts. Click a “plus” sign in a craft window and select the friends to whom you want to sell the invitation in the appearing pop up. To become your helper, all your friend has to do is open the game or accede to your request in the mail.
ATTENTION! You can be a helper only in one craft currently performed. To invite a friend to a craft again, you need to complete the craft you have previously hired this friend for.