I am considering outsourcing the development of an iOS app to various independent developers. I will have have various testers of the app. We all work for separate companies. Some of these testers will be customers, who I would like feedback from. As there are multiple developers involved I expect there to be a new release on a daily basis.
How can this be done? Would each of the testers need to buy some sort of license to avoid having to go through the app approval process? Is there any smooth way to do this so that it will not be a hassle for our friendly customers, who are willing to test our app?
Take a look at TestFlight. You'll enroll in the iOS developer program, have your testers set up their devices as members of your team and be able to push new builds to TestFlight. Your testers will receive notifications and get the new builds to test.
Any developer who wants to build and test their code on their own iOS device without needing to send the app to another developer to codesign the app for them (or using their company's certificates) needs to enroll in Apple's iOS Developer program ($99/annum).
For company teams using less than 100 max total iOS devices, only the company or team leader needs to pay and enroll, but then that one person needs to handle all the accounting and management (registering everybody's device, renewing and sending out certificates and provisions to everybody else on the development team, submitting apps to Apple, etc.). Test devices belonging to testers can be among those 100 max total device. See Apple's documentation on Ad Hoc distribution.
The current way users can access and test an unpublished iOS app is through the TestFlight program. If I may answer the rest of your questions using the Apple page, it says:
To take advantage of TestFlight, you’ll need to upload at least one beta build of your app to App Store Connect, and invite testers using their email addresses or by sharing a public link. Testers can get started by accepting your email invitation or following the public link. To install your app and provide feedback, testers will use the TestFlight app for iPhone, iPad, iPod touch, Apple Watch, and Apple TV.