Alejandro,
Thanks for the reply. There was a mistake there, but unfortunately it was not the actor-id element, it was that I had not put the "class" on the assignment. Alfresco has defined a custom assignment class which (as I understand it) allows the use of user names in the assignment and (I assume) it validates those users and puts the proper actor-id values into the swimlane. Either way, from all of the example code and the built-in workflows this is correct. So, I changed my submission from:
<swimlane name="initiator"></swimlane>
<swimlane name="Content Providers">
<assignment actor-id="Content Providers">
<actor>#{bpm_assignees}</actor>
</assignment>
</swimlane>
to
<swimlane name="initiator"></swimlane>
<swimlane name="Content Providers">
<assignment actor-id="Content Providers" class="org.alfresco.repo.workflow.jbpm.AlfrescoAssignment">
<actor>#{bpm_assignees}</actor>
</assignment>
</swimlane>
This doesn't reflect a change in my workflow, just this message (I must have removed the class accidentally when I simplified it for the forum).
Thanks again!