Are some of the people completely new to machine sewing? If so, I suggest having them do some learning exercises with scrap fabric or even paper. When I learned to machine sew in middle school home-ec class, we did some learning exercises with paper as our "fabric" (no thread in needle or bobbin) - with lined paper we stitched along the lines to the hang of straight line sewing, and with some pages from a children's coloring book (simple large drawings), we learned how to handle curves and points. (If you don't want to use paper, you could do the same excises with using a pencil to mark lines on light colored scrap fabric.)