Activiti currently supports the default BPMN users: humanPerformer (owner/assignee) and potentialOwner (candidate groups/users). The BPMN standard xml elements can be used as an alternative the Activiti extension attributes. It appears that the humanPerformer is mapped to the assignee and the potentialOwner is parsed and mapped according to the identity type (user or group).
Additional IdentityLinks can be added with custom link types via the TaskService:
However, there is no mechanism to support custom link types as extensions in the BPMN template. We propose extending the TaskDefinition class to support parsed custom resource extensions. In addition, extending the UserTaskActivityBehavior class will assign the custom values to the existing TaskEntity IdentityLinkEntity list member.