Parties
A party keeps friends together across matches. The party bar is at the bottom left of the main menu, with room for 6 players. You sit at the left; click a + slot to invite a Steam friend. Hover a member for options such as kick and promote.
The Social menu lists your friends, whether they are online or in a game, and parties you can join. Search can also filter by online, offline, in game or in a party. Who may join your party (open to friends, or invite only) is set in Settings.