If it's like most offices... fluourescent lighting, which really sucks for photography... ugly greenish yellow color and it cycles on and off, so you will have a lot of missed exposures, too. It's even worse if you mix it with outdoor light and/or flash.
Buy a flash.
High ceilings and big spaces... forget bouncing the flash. So also get a flash bracket and an off-camera shoe cord. (There are cheap brackets and cords on eBay, if you don't want to spend very much.) These allow you to use direct flash and get nice looking images. Optionally, get a diffuser to use on the flash.
Or reposition your subjects closer to windows and use natural windowlight. If you combine flash with windowlight, you'll need to put a colored gel on the flash so that it's color is better matched with the windowlight. Otherwise the mixed color temps can be a problem.
But, basically, you don't need a lens. You need a flash.
270EX is quite small, mostly automatic modes only, might be sufficient for your purposes. 320EX is a little lbigger, has a few more controls for the user and also has a continuous light if you want to shoot video with it too. Canon 430EX II is compact, reasonably affordable, but still pretty powerful and flexible. Now discontinued but still available - 580EX II is bigger, a bit more powerful and more expensive, and gives you a lot of controls, if needed. 600EX RT is probably way more than you need, powerful and ideal for multi-flash setup and wireless off-camera flash contro... it's also quite pricey. (430EX/420EX and 580EX/550EX all also can be used wirelessly off-camera, but use an older near-IR-light form of communication - rather than radio control - that requires line-of-sight and limits setups.)