The trust that people feel in their social groups is linked to important social outcomes such as member satisfaction and collective task performance. To understand the behaviors and conditions linked to trust, past studies of trust in groups have typically relied on cross-sectional surveys, but these are limited in their ability to identify causation. To better test the potential causal pathways between trust and behaviors or group properties, we paired a two-wave longitudinal survey of 2358 participants in Facebook Groups with logged activity on Facebook. Using latent change score modeling, we examined how trust may predict changes in behavior or group properties and how behaviors and group properties may predict changes in trust. On one hand, people who trust a group tend to contribute more written content to the group over time; and while groups that are more trusted tend to add more administrators and moderators over time, groups that have many administrators and moderators does not tend to be trusted more over time. On the other hand, people’s trust in a group increases over time when the group is well-connected and active overall, while that trust decreases over time when that person is also actively involved in multiple other groups. These findings suggest a positive feedback loop related to active engagement and trust: seeing activity in a group drives trust, which in turn leads to increased individual activity and hence greater overall activity in the group. Overall, trust may be best promoted by encouraging both active engagement and friendship formation.