Gold bonus page or sometimes on the Homepage during special promotions.
Friend can be invited by accessing Yahoo or Google mail and sending e-mails, sharing your personal link on twitter or facebook, or simply copying the link and posting the link wherever you wish.
Every 10 invited citizens who reach level, the inviter receives the Society Builder achievement.
Every new citizen will earn additional rewards for the citizen who invited them. An alert will be received for every reward earned that way. More info about bonuses on Gold bonus page.