Case roles can be assigned at case start, as part of Start Case modal dialog.

Although checks of each role's assignment input and it's cardinality are run successfully in the UI, the backend is using the case file mechanism for adding users/groups to case roles.
The problem with this approach is that case file API doesn't support adding more than one user and one group per case role as it uses java.util.Map to store assignments.

Although case roles re-assignment feature can be used to amend the assignment list after case start, this workaround won't work for Ad-hoc Autostart tasks, which have already been started and hence are not affected by case role re-assignments.

One solution to this problem would be replicating the existing re-assignment mechanism we have to assignment mechanism at case start. This might be easier than making changes to the case file API, which on the other hand definitely needs improvements in this area.