How to give a crash course?

Some friends of mine always want to take my pics when I take theirs, but it's hard for me to set up the system for them, explaining them how things work also isn't quite easy. As a result I usually end up disappointed with messy out of focus results.

Since it's portraiture, I set the camera into manual mode, f/2.8, 1/2*focal length, auto ISO. That's completely fine.

However, where I struggle the most is autofocus. Wide open at f/2.8 depth of field is already extremely thin so missing focus can happen even to me. What focus mode should I give to my friends? (Also tried with f/4 but some shots were still out of focus)

I have a D7100, so live view kinda sucks (but facial detection works reasonably well), AF-C with d-9 is kinda my go to, maybe even set to full auto for picking focus points? Thus far I've given them single point and told them to move it around and compose. None of them did... In each photo I was exactly in the center where the focus point was. They seem to end up overwhelmed by the focus points they completely forget about composition.