In some quests, you may be asked to help or ask your friends for help.
   
You can send invitations to your friends to join you in the game by clicking the "Add Friends" icon and then "Find Friends".
   
If your friends are already playing the game, you can add them using the game ID at the bottom of the game screen.

You can also check this little tutorial here:

https://www.facebook.com/ci2tropicalstories/videos/353886459009730