Back in the olden days you would register individual users' devices (UDID) with your profile and then build an .ipa with manifest and upload these to some private website. All they have to do is surf to that website with their mobile device and install the app. The downside is that you have to prepare a web site that is secret to those users. You can also use Apple Configurator 2 but it requires users to use a Mac to run it.

TestFlight is very easy to handle. Upload a build and start a beta test. Add only those individual users who should have the app. Downside is that the beta test expires in 90 days so you need to upload a new build every 90 days.

Overall I think the AdHoc solution (see above building the .ipa) is best but you have to update the build each time you add a new device.