Phil,
I'm sure you can understand that Apple is unlikely to assist you in "transferring" a purchased app to another device. That is the sort of "old thinking" that came with purchasing an application for a computer on hard media and then loading it onto another computer. That has always been a problem for software vendors since it's difficult to police such activity and protect against piracy. So, as Kaykaykay points out, you'll have to purchase a copy of each app unless the iPads are on the same iTunes account. (And even then there is a limit.)
However, if you're doing what you describe, i.e. evaluate an app and then purchase individual copies only for the iPads you want your employees to use, there is a "workaround" of sorts. You can purchase a particular app, evaluate it (preferably immediately), and then request a refund from the iTunes App Store for the app you've purchased. Trying to do this can be a chore. (Apple doesn't make it easy to figure out how to do so.) But in my (limited) experience I've been able to return an app that failed to provide the functionality I required. The charge to my credit card was reversed within a couple of days. (Try wandering around in the maze of iTunes App Store screens on a computer to figure out how to get in touch with iTunes App Store support.) Once you've done this, you can then purchase the app (if you wish) for each of the employee's iPads using individual iTunes accounts all linked to a single credit card.
Finally, there is apparently a way to purchase apps in bulk for multiple iPads. This service may be problematic for you since it's designed, as I understand it, to deal with large corporate customers. But you may be able to find some help from an employee devoted to small business sales at your local Apple store, especially if you plan to purchase numerous apps. I'd recommend that you contact your nearest Apple store and ask to speak to the "genius" who has this particular responsibility.
