possibly the most noticeable difference between the two will be the paint bucket tool.
mac su uses the OS X color picker and if your friends are familiar with their computers then they'll be familiar with the color picker.. unfortunately, SU modifies the color picker so that a list view of the materials aren't given in mac SU and you have to rely on thumbnail views of materials or popups for the actual names.
there are also differences in which modifier keys to use with different tools.. namely the cmmd and option keys on macs being different than ctrl and alt on pcs.
see these reference cards for details.
http://dl.google.com/sketchup/gsu8/docs/en/SketchUp8RefcardMac.pdf
http://dl.google.com/sketchup/gsu8/docs/en/SketchUp8RefcardWin.pdf
there are actually a few things you can do in mac SU that you can't in windows but since you're training from pc to mac, i don't think it matters much at this point.
re: setting up the workspace.. the toolbars work differently on macs and pcs.. all of the toolbars on mac will be floating unless the tools are added to the custom toolbar at the top of the su window.. that said, as far as teaching sketchup goes, it shouldn't matter much and your friends can just arrange their toolbars as they deem comfortable.
all in all though, sketchup is very similar on both platforms.