How To Return Multiple Usernames?

What technique could be used to return multiple usernames from a called script to a workflow?  For example, we would like to return all the usernames of a specific key member type on Project numbers associated with various distributions lines on an invoice.  We send FYI emails to them so they know to review the invoices as needed.  It would be a different number of names on each invoice so I don't want a variable for each name since I don't know how many there would be. 

Should I be able to loop through the workflow calling the same script multiple times with counters to keep track?  Or is there a better method?