In Brawl Stars, you can play in pairs or in a three-person team. If you choose to play alone, you will be automatically assigned to other players. On this page of the guide ti Brawl Stars, you will learn how to add people to friends' list.
After going to the Friends tab, you will see the following window. At the bottom, you have two options - sharing code and joining with it. If you create a code, you can share it with your friends.
You can also join their team if you get such a code. Once you are in the same team, just click on a friend and add them to your friends' list.
If you want to add someone you have on Facebook, just connect your game to your account. You will get access to the entire list of your friends. You can add them to your friends' list and chat with them.
Friends from the game
The game encourages you to find new friends, so you can invite friends of all your allies with whom you have played. Just go to the "suggested" tab - the game will show you the list of all your potential friends.