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.
