You must launch Dota 2 in Steam and get ready for the lobby invitation before the game starts.
A Lobby Invitation will pop up in Dota 2, click [Accept] to enter the lobby.
Make sure your setting didn't block the player or block invites from players not on Friends List.
If the Accept button is dimmed or you click Accept and nothing happens, please wait patiently for the system to resend the invite again. Contact firstname.lastname@example.org if this issue continues.