You can manage your friends by tapping the two-person icon at the top of the main screen.

The Friends tab shows all of your current friends and allows you to remove a friend (trashcan icon), see if a friend is online (eyeball icon), send a message (chat icon), and send a friend XP (XP icon).

In the Social Friends tab, you can see all of your friends from your linked accounts.

Under Recent Teammates, you can add a player from a recent match by tapping the yellow add friend icon.

If you know the name of a player you want to add, go to Add Friends and use the filters or search to send a friend request.

To accept or refuse an invitation from someone, tap Requests at the bottom.