Multiplayer party systems allows players to join a group that plays online games together, either through co-op or matchmaking.
Gamespy was a system that allowed players to do this with numerous games with this capability. The Dreamcast was the first console to allow an online party system in some games, most notably Phantasy Star Online. Then Xbox Live for the original Xbox allowed a party system in some games, most notably Halo 2. Some PS2 games also had party systems; SOCOM was a popular game with this feature. The Xbox 360 and PS3 featured more multiplayer party system games with increased online capabilities.
The New Xbox Experience allowed for a party system give 8 players voice chat. That enables an 8 player party in a game to communicate with each other and not the other team, as in a 16 player big team battle match in Halo 3. Normally, you could hear the other team, but with 8 players in a party versus 8 other players they can communicate without interacting with the other team, as ranked matches do in game.
Each game with a mutliplayer party system is usually different from other games. Games such as Perfect Dark Zero allow players joining the party or leaving the party without interrupting gameplay. With games like Halo 3 players cannot join during matchmaking games or be replaced by new players as is done in Perfect Dark Zero. However, custom games can be joined on the fly.
Agreement on what type of game to play differs in games as well. In Halo, the leader controls the game but the game can be delayed by the rest of the party. In most other games the leader selects the game, but the rest of the party must be "set" or "ready" or else the game cannot be started.