This is more of a note for myself. For an activity, this will get the affected user of the parent work item.
$Context/Path[Relationship='CoreActivity!System.WorkItemContainsActivity' SeedRole='Target' TypeConstraint='WorkItem!System.WorkItem']/Path[Relationship='WorkItem!System.WorkItemAffectedUser' TypeConstraint='System!System.User']/Property[Type='System!System.User']/FirstName$
Obviously it is only the first name and the references will need to be changed to match the reference alias in the MP.
