when taking pics of a bunch of people...

says like a football team where there are football players in rows
which f-stop do you use to make the people in the back and in the front rows all in focus?

f8?
f11?
or f22?

which f-stop is best for a situation like this?

Depends entirely on the focal length and distance to subject, as well as the lighting situation.

What Mike said. The shorter the focal length, the wider your aperture can be and still get everyone in focus. Use the depth-of-field preview button on your camera, or use the DOF calculator here:
http://www.tawbaware.c​om/maxlyons/calc.htm

if you are unsure, use the smallest stop that will still give you enough shutter speed. amount of light is another factor.

Also depends on if you are indoors or out.....using strobes...?

Of your 3 choices, you will never need to use f/22 for a big group shot like a football team. You will never be able to get close enough to them that your distance to subject will narrow your DOF enough to need it.

f/8 or f/11, assuming you have enough light, should work just fine for about any group shoot situation.

There are trade-offs between sharpness from being in-focus to softness due to lens diffraction (google Lens Diffraction to learn more). In general, except for special effects, you will probably never want to go beyond about f/11. Your optimal sharpness will generally be about f/8 or f/11. After that your image will begin to go soft due to diffraction.

Nobody can tell you exactly what to use, but you can figure out how wide the players will be staged and use that to figure out how far away you'll need to be to fit them all. From there, you can start plugging values into a DOF calculator to get a ballpark number. You could also google image search for some images similar to the ones you're going for and hope they have EXIF data in 'em.