Michael Condrey has tweeted out that the team at Sledgehammer Games has heard fan-feedback and will be enabling parties in Gun Game in Call of Duty: Advanced Warfare in a future update. As of now, the game mode is constructed as a Free-For-All mode, which results in parties being disabled.
@charlieINTEL yeah, the people of spoken. Will look to make that change in a future Gun Game update.
— Michael Condrey (@MichaelCondrey) February 17, 2015