Is there a function that can be used in Reports that will return a User's Alias based on a ranking t

I am trying to create a report based on Activities that counts how many times various Users have modified Tasks in a given month (as a measure of User adoption). For each User there will be a column and I already have the logic that identifies if a given User updated a given Task.

The problem is that for each column, the logic works on a different User's alias. That is fine, it works, but requires re-coding if the Users change.

Is there a function which will return an alias based on a ranking that does not change with User details? Something like: JoinFieldValue('<User>', 1, '<Alias>') , JoinFieldValue('<User>', 2, '<Alias>') , JoinFieldValue('<User>', 3, '<Alias>') ?