Organisers of crowd mapping initiatives seek to identify practices that foster an active contributor community. Theory suggests that social contribution settings can provide important support functions for newcomers, yet to date there are no empirical studies of such an effect. We present the first study that evaluates the relationship between colocated practice and newcomer retention in a crowd mapping community, involving hundreds of first-time participants. We find that certain settings are associated with a significant increase in newcomer retention, as are regular meetings, and a greater mix of experiences among attendees. Factors relating to the setting such as food breaks and technical disruptions have comparatively little impact. We posit that successful social contribution settings serve as an attractor: they provide opportunities to meet enthusiastic contributors, and can capture prospective contributors who have a latent interest in the practice.