Invite friends option can be found on Store->Gold Bonus page.
Friend can be invited by sharing your invitation link.
The citizens you have invited bring you a 10% bonus from all the Gold they get from game - achievements, level ups or Gold purchases! Help and guide your friends through their progress,and with their achievements you will be rewarded too!
Whenever your refer hits level 20 you will win 5 gold and also the Society Builder Medal. If you invite 25 friends and help them reach level 20 you can request for the ambassador medal and as a reward you will get 50 gold.
Every new citizen will earn additional rewards for the citizen who invited them. An Alert will be received when someone registers under your refer link. More info about bonuses on Gold bonus page.