As gaming spectatorship has become a worldwide phenomenon, keeping the spectator in mind while designing games is becoming more important. Here, we explore the factors that influence spectators’ engagement. Through the use of GRiD Crowd, a game akin to life-size Pong, different levels of spectator influence on the game were tested and their impact on engagement via arousal measures were analyzed. Spectator influence on the game was accomplished via smartphone, where 78 participants put in different audience compositions (alongside friends or strangers) were tested. We found that when the spectators had an impact on the game, higher levels of emotional arousal were recorded, which generated an increase in engagement. These results provide a suggestion of design that could be used by game designers who wish to engage their spectatorship, a segment of their target market that is becoming impossible to ignore.