You can access in-game friends via the Social Tab found in the top right corner of the main screen.
Select the FRIENDS tab to view your Party, and your Friends list.
Your Steam friends, who played the game at least once, will be automatically added to your Friends List in-game.
From there you will be able to see the ONLINE/OFFLINE status of each friend.
Selecting a friend will open a menu allowing you to add them to your PARTY, as well as perform other actions.
ADDING FRIENDS FROM WITHIN THE GAME
While in-game you have multiple options to add players to your friends list.
Adding a player as a friend in-game means adding them as friend on Steam.
From the scoreboard
When looking at the scoreboard click on found towards the end of the row.
This will bring up that player's Steam profile, click on Add Friend to send your request.
Once the request is accepted, the player will automatically appear in your friends list.
From the end of match summary
Click on the player you want to add as a friend to access the drop-down menu, click ADD FRIEND to bring up the player's Steam profile and add them.
From your Party
Click on the player you want to add as a friend to access the drop-down menu, click ADD FRIEND to bring up the player's Steam profile and add them.