How to play a cooperative game is still the same friend?

[stanisaurus]

How to play a cooperative game is still the same friend. I see a friend, and he'll invite me too sees the invite and then what?I miss the option to play there or something.When you give online play of course always with someone else.

[_I_]

you can go to steam, click on the chatbox for your frined, then join, or open the friends chatbox from within portal2 and click join

[stanisaurus]

Thank you for your help.