Symptoms
Workflows not initiating for certain users, despite the users being in the correct group to have design/participate privileges
Diagnoses
The site collection name had been changed after provisioning Appit on the site, causing an inconsistency in the SharePoint group names and the references to these groups in the context of Appit.
Resolution
Rerunning the registration wizard updated the group references to the current SharePoint groups, allowing all users in the Design/Participate group to initiate workflows.