Most likely your problem is too shallow depth of field and an uneven plane of focus.
Group portraits can be particularly tricky. If you use a large lens aperture thats rendering shallow depth of field, you have to make sure all the subjects' faces are lined up on the same plane of focus (which is parallel with your camera's sensor plane, unless you use a special Tilt-Shift lens to change it).
Alternatively, or maybe in addition, select a smaller lens aperture that will increase depth of field and allow you to get all of your subjects adequately in focus. To be able to do this and still have a high enough shutter speed (even on a tripod), you might need to use a higher ISO. When photographing people, even using a tripod you often still need to keep shutter speeds up to some degree, in case they move slightly during exposure. The tripod (and Image Stabilization on some lenses and cameras) can help with camera shake.... but can't help with subject movement.
Blur due to subject movement can be mistaken for focus problems. So just be sure you know which you are dealing with, or your corrective efforst might not help.
In time you will learn to estimate what aperture is needed in particular situations. Depth of field is controlled by the lens aperture and the distance to the subject. Lens focal length is a secondary factor, in that it in part determines your working distance (though depth of field doesn't actually change with changes in focal length).
A wider angle lens can help, but you then have to start to watch out for optical perspective distortions that can look particularly odd with portraiture. Too close to your subject with a wide lens and you'll see exaggerations, where their nose looks big and their ears look small.... and where people nearer the edges of the images appear to be anamorphically stretched.
Some things I'd suggest are to study up on depth of field, get a depth of field calculator and use it, and to pick up a copy of "Understanding Exposure" by Brian Peterson.... it's a very helpful book that discusses these and many other factors in much more detail than we can offer here.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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