Pieceable is really neat, but it's just not gonna work for most apps that use any of the phone's hardware and it doesn't perform super well.
TestFlight is almost perfect. Lots of times people can't figure out how to register a device and then provisioning profiles get messed up and the app doesn't work for them. It's as seamless a process as you can get and still work within Apple's system. Once you get everything set up, it's quite amazing.
One last way that's super easy for users (but probably not supported/allowed by Apple) is to use the Enterprise distribution. This costs 300, instead of the normal 100 and it lets you sign your own apps for use inside of your "company." Once you get the Enterprise distribution building, you can put the app on a website and have anyone download and use it. Unfortunately, if Apple finds out about this they can ban you from the developer program. If you put it behind a password so only people you want to find it can, you'd probably be fine. I'm not sure what it takes for beta testers to count as part of your organization.