so you are in a round playing solo when 1 of 2 things happen. 1) you get an invite to play a multi with a buddy of yours. 2) you see a buddy is now online and you want to quit so you can fire up a new multi player game.
either way, you want to click "menu: end round / main menu"
now you can either accept an existing invite or create your own multi player game from the main menu.
gl