You can add friends and check the list of added players in the Neighbors menu. To open it, click on the "Social" button (a button with two men in the bottom menu), and then on another button with two men.
You can add friends in the following ways:
1. To add a random player, open the first tab and click the Confirm button. After sending the invitation, the selected players will go to the waiting list (second tab "Requests") until they accept your invitation.
2. To add a specific player, open the "Add a Friend" tab and enter their code. You can use 5 friend codes per day.