How to invite your friend for Co-Op

Load The game
Push enter to get to the main menu
Pick a game mode
Click to start a new game[and play until 'Rhonda's Garage'] or continue[if you alread have a save]

Load some more
shift+tab into steam overlay
Oppen friends list
[Optional] Message them[the friend you wish to invite], and ask if they want to play
Click the drop down arrow, next to the friend you wish to invite
Click invite to game
Wait for them to join
They pop in
Kill zombies together