Playing the game with random team members is fun but forming up with two of your friends to form a highly efficient trio is even more fun. Luckily, there are some tools in the game to help you make this happen.
To invite a friend, first tap on the “My contacts” button on the bottom right of the screen
This should show your current Friends list as well as friends currently live on Steam, if it is empty, don’t worry as adding new ones is fairly painless. Just type your friends name as it is shown to them in the game into the section on the bottom of the tab and press “ADD”
This should prompt them to get an invite, and when accepted they will appear on your friends list. If they are online, just tap on their name and invite them to join your team.