The other day I was trying to take a picture of all my family spread out. When I put the image onto the big screen, the people in the middle were in focus and those around the edge weren't in focus. How can I make sure everyone is in focus?
A few suggestions:
Use a small aperture. An aperture with a large f-number (eg. f/11 or f/16) has a small opening. ('f' means 'focal length', so 'f/16' means the focal length divided by 16, or 1/16th of the focal length, where as f/2 is one half of the focal length, which is much larger than 1/16th - which is why f/16 is smaller than f/2). A small aperture has a large depth of field making it more likely to keep everyone in focus.
When framing your shot, don't put people near the edge of the frame. Rather, take the shot expecting to crop it down to the people. This will keep your subject matter near the centre of your image where your lens is sharpest. However, you will lose resolution. Depending on the camera's sensor size the final product may not be of the quality you need.
Research your lens on the web to find out at which aperture it shoots sharpest, and use that aperture.
Are you talking side-to-side edges, or front-to-back? As Michael Clark says, some lenses are sharp in the center and softer at the edges. Excluding special effect lenses, this is undesirable, and edge-to-edge sharpness is the ideal. Better quality lenses have better edge-to-edge focus.
If you have issues with images that are softer at the edges than in the center at the same focus distance, you might need to invest in a better quality lens.
Front-to-back focus differences are a major consideration. If the people on the edge of the frame were closer to/further from the camera, they might be outside of the depth of field for your camera settings.
Try positioning your subjects so as to limit the front-to-back distance between the closest and furthest person. Ideally, put everybody in a single row.
Also use the smallest aperture you can. There are focus charts that will tell you the depth of field for your lens for a given aperture and focal length. You'll also want to take test shots with the settings you plan to use in a photo shoot and review the results for acceptable sharpness.
Use f/11, bring a big flash, and don't pixel peep.
f/11 will have a deeper depth of field which means you can have people behind each other and both rows will remain in focus. The big flash will help you keep your ISO and shutter speed down. Despite these steps, if you zoom into the image to the pixel level, you will still find some differences in sharpness. Realize that's inevitable with people are spread out.