If you want to play Duos or Squads with your friends you will be required to add them. To do this you bring up your 'Friends' menu by tapping the person icon located at the top right of your screen. The 'Friends' menu will display people you matched up with during your previous game, and other players regardless of whether they are online or offline. It is also here that you will be able to open up your 'Friend Requests' and accept any that may be pending.
To expand your directory of friends you have to click the 'Add' button to bring up a menu of recommended players. It is here you can send a 'Friend Request' to these people, if however there is someone specific you want to add you will need to search for their unique player tag to send them a 'Friend Request' where you will then have to wait for them to accept it.