Party Play is a game mode introduced in the 2009 game, Guitar Hero 5. It allows the player to let the game play music in the background, while they go and do whatever. If someone wants to play the game, they are free to simply jump-in and begin playing. When he/she is finished playing, they can simply drop-out. As the name suggests, this mode is great for parties, as it works as a jukebox, and plays random songs continuously. The music also never stops until someone turns off the game.
Party Play can be activated in DJ Hero by going to the song of your choice, and then holding down the Euphoria button once the note highway appears. The mode should engage once the first note reaches the strike line.
The DJ Hero version of Party Play works in a slightly different way than in Guitar Hero, rather than being a separate mode altogether, one only needs to go into Quick play mode, and create a setlist/play a song. The mode also does not work like a jukebox, automatically choosing random songs; instead, the player is required to create a setlist.
The mode works more along the lines of an, "Autoplay," mode.