iTunes Connect is a bag of hurt sometimes, especially if you have clients that you work with. One of the biggest frustrations that I have (and hopefully this will be changing in the future) is when you need to submit builds and you cannot use an email for an Apple ID in iTunes Connect to manage your client’s Apps, TestFlight, Users, etc.
While there is no easy way to accomplish this yet, at least there is this post by mokagio that helps ease some of the pain…
TL;DR You can’t! There is no Apple way to do it, and this is yet another frustration of working in this platform. The steps described below are just a workaround to handle multiple iTunes Connect accounts in a sane-ish way.
While I don’t love using the Application Loader, at least it works. Also, creating email addresses for Apple IDs with the “+clientname” trick works great if you use a Gmail-based email system, and I’m fine with having my clients use this method when they add me to their iTunes Connect accounts.