In this paper, we present crowd experience as a novel concept when designing interactive technology for spectator crowds in public settings. Technology-mediated experiences in groups have already been given serious attention in the field of interaction design. However, crowd experiences are distinctive because of the spontaneous, uninhibited behavior exhibited. In crowds, extreme sociality and the experience of performing identity in public emerge spontaneously. By bridging crowd theory and pragmatics of experience, we establish an understanding of crowd experience as a distinct sociality within interaction design that unfolds through imitation and invention. We deploy that understanding in an exploration of spectator experiences at three football matches in which an experimental prototype, BannerBattle, was deployed. BannerBattle is an interactive banner on which spectators can grab space in competition with their rivals. The more noise and movement they make, the more screen real estate they gain. BannerBattle therefore enabled us to explore the emergence of imitative and at times inventive behavior in enriched crowd experience, by augmenting and supporting spectator performance in this way. We discuss the value of a conceptual understanding of crowd experience for technology as an unexplored potential for designing new interactive technology at spectator venues.