Activity List Inquiry

I'm modifying one of our current activity lists and had a question on how to expose the users associated with the activity. The owner of the activity will use this information for sorting purposes. For example, the owner of the activity list is John - the list shows all of John's Open Tasks. John is on the sales team and would like to sort the tasks by Partner (there are typically two partners assigned to each task).

Previously, John exposed the Delegated By field - he assigned one Partner to this field. The problem with this, however, is that there are multiple partners associated to each task, so exposing one does not solve the overall problem. Is there a way I can expose multiple users of a task and make the tasks sortable by user? Below is a screenshot of the current layout: