Just to let you know, AirportCityGame will be moving to superfast new servers very soon. This will stop the slow running and downtime you may be experiencing when you visit.
For more information - Please see THIS thread.
How do I add friends?
1. To add a random player open the first tab → press the confirmation button. When the invitation is sent the chosen players go the the waiting list (the second tab) until they accept your invitation.
2. To add a certain person open the friend codes tab and enter your friend's code. Please, note that you can redeem only 5 codes a day.