I have a Canon T3i and am just venturing in to new lenses..I have been asked to do indoor team and individual soccer pictures..they will be taken on a gray backdrop with white umbrella studio lights..what would be a good lens to purchase or rent?
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.
I am using a Canon 28-300 and a 7D Mark 2. The 70-300 has good detail throughout the lens. Personally, I wouldn't use a backdrop. I would use the environment, the green of the turf with the lines in the background. Foot on a ball kind of pose. And then while you are that get some action shots too. Get all the standard shots like a coach coaching, kids kicking, goalie defending a shot. Above all else try to define your style. The last thing I want to hear about is how someone else used to do it.
Get a test subject and get your settings dialed in and then tell them when you are ready to start taking the money shots.