I've been using manual focus lenses. In a few cases, I wanted to be in a picture, but it takes too long to explain how to focus. Even people who have used manual-focus lenses in the past, who should know how to focus, can't get it right. Is there a setting or technique I can use to get correct focus when handing the camera off to someone else?
You've probably already figured out that you should prefocus, and don't mention that it's manual focus or you risk having it changed inappropriately. But the camera is changing hands, and people are moving about, so how do you get the focus right?
This is where your knowledge of depth of field comes in. Stop down the aperture until you're reasonably confident that you'll be within the depth of field when the shutter is fired. Consider using a stop smaller than you think you'll need. If you really have no idea and lighting is good, try the hyperfocal distance at F8.
With less lighting, such as indoors, you should still attempt to prefocus and stop down the aperture, but you'll be limited by ISO and shutter speed. In these cases, if you increase distance and decrease focal length you can further increase depth of field. You can always crop later, provided the image is acceptably sharp.