The Sims Mobile allows you to meet sims controlled by other players. Contrary to NPCs, these sims can throw parties, invite you to become friends and expand their homes. Each of the sims controlled by players have a special marker above their heads. See the screen above.
Adding to friend list
If you meet an interesting sim, whom you want to add to your friends list, you just need to click it, and an appropriate option will appear. However, you cannot be participating in any event at that time.
If you're invited by another player, you'll get a notification in the messages. You can accept or reject an invitation.
If you want to invite a friend, just ask them for the friend code. You can find the appropriate window in the friends section. It's enough to input the code and send the invite.