On a task report, you can look under 'Primary Assignment' for a field called 'Assigned by ID' which will give you the ID of the person who made the assignment to the Task Owner. You'll need to use text mode to the translate that over into the readable name of the person. If your tasks have multiple assignees, and you want the same information for every assignee, that would require more complex text mode but is do-able. I don't have one that does that to share quickly, but I can mock it up if you need.
linkedname=primaryAssignment
namekey=view.relatedcolumn
namekeyargkey.0=primaryAssignment
namekeyargkey.1=assignedByID
querysort=primaryAssignment:assignedBy:name
valuefield=primaryAssignment:assignedBy:name
valueformat=HTML