How to create a report on tickets "worked" per agent/analyst?

I'm looking to create a report for our IT Operations director that will show how many tickets are "worked" by an IT agent or analyst. Reporting on the number of tickets created or resolved is easy enough. However, we're looking to account for those tickets that weren't created or closed by the staff member, but they worked on at some stage. The general idea we are looking for is to track tickets the IT staff have put an additional comment or worknote into.

To further complicate the idea, we want an individual count per ticket. I don't need to know Bob created 10 notes on INC0015872, just that he did work on it at some point.

Any help or suggestions to point me in the right direction are greatly appreciated!