Your camera probably came with an 18 to something lens, possibly the EF-S 18-55mm f/3.5-5.6 IS II. That lens should certainly serve you well for what you want to do; use it at 55 mm for the individual portraits. It's slow at the long end, but that's okay when using flash, and an ultra-narrow depth of field is arguably not really appropriate for football player portraits anyway.

That said, if you plan to take more portraits in the future, you may want to buy a dedicated fast prime lens. Both 50 and 85 mm are good focal lengths for portraits on APS-C. Which one is better depends on the type of portrait you want to take (head only, head and shoulders, full body), how far away from the person you can stand, and your personal taste. You can perhaps use your zoom lens to find out what focal length you prefer. The EF 85mm f/1.8 USM is an excellent lens, and so are the 50 mm f/1.8 lenses Canon has made over the years. These are all popular lenses that have been around for years, so you will be able to buy them used for reasonable prices.

For the group shots, try not to go too wide, certainly not wider than 18 mm. It would distort the players at the edges of the image. Rather, position yourself as far away from the team as possible and use a relatively long focal length. Again, an 18 to something kit lens should serve you well.