It depends on what they did for you. If they washed your glassware and cleaned up the lab, they dont need to be co-authors. If they did any experiment themselves that is included in the paper, they should definitely be co-authors. If they did some work that was integral to the understanding of the research, but not really included in the data, its up to you.
Also, I agree that the person who does most of the work/writing is listed first, and the PI listed last.