Crowdsourcing refers to the use of technologies to gather the collective effort and wisdom from an undefined group of online users for organizational innovation and/or problem solving. A critical challenge for crowdsourcing users and providers is to engage online participants to make sustained contributions. This research in progress paper proposes a behavioural perspective on the definition and measurement of participant engagement in community crowdsourcing, a crowdsourcing model where all participants can see and react to everyone else ideas. In our research, we propose to conceive participant engagement as a set of engaging behaviours and evaluate it through the magnitude, temporal intensity, diversity, and recency of these behaviours. We illustrate the construct operationalization by the Participant Engagement Index (PEI) that quantifies engaging behaviours in the context of Mind-Mixer, a community crowdsourcing service provider. We further provide an initial illustration of the PEI’s utility through the analysis of field project data from MindMixer. We expect that our study will provide guidance for future research into existing and new practices to study and improve the active and sustained participation of crowds in open collaboration forums.