I am not an expert at this, but I do development with my personal developer account, as well as work on iOS applications at work.
Please look at this link...
https://developer.apple.com/support/compare-memberships/
There is a difference between individual and organization accounts.
An individual can only enroll in the "Apple Developer Program (individual type)" as an individual and can not invite other people to the account.
An organization (company) may enroll in either the "Apple Developer Program (organization type)" or the "Apple Developer Enterprise Program" or both (each will need a unique AppleID however).
Only the orgainization version of the "Apple Developer Program" will allow inviting other programmers/developers to your "team" to develop and sign an app on your behalf.
The individual "Apple Developer Program" lacks this team feature as it is not needed for personal development.
Then there is the difference between "Apple Developer Program" and "Apple Developer Enterprise Program."
The difference is that the "Apple Developer Program" allows developing apps for release on the app store and is what applies to most developers.
The "Apple Developer Enterprise Program" is designed for companies that want to design an app for internal (inside their organization) distribution. It does not allow releasing apps to the app store, it is only for internal distribution of apps.
It is possible for a company to enroll in both the "Apple Developer Program" and "Apple Developer Enterprise Program" at the same time, if you need to distribute an app internally and on the app store at the same time. However, with an internal application, since it is not on the app store, guidelines are more relaxed on what you can and can not include since the app is not checked by Apple. As soon as it is signed it can be distributed (though a companies own distrubution system).
One other thing to mention, as of recently enrollment in the Apple Developer Program (that requires a yearly fee) is no longer required for personal development using your own personal device (iOS device). Enrollment will only be necessary to submit to the app store.
I am not sure about the requirements for testflight though, someone might be able to answer this one.
If you are doing development at a company you should never need to give your AppleID information out to the developers. They can be invited to your "team (Apple Developer Program or Apple Developer Enterprise Program)" and develop/sign apps with organizations's account. If you are using a personal (individual) account at the company I would highly recommend contacting Apple to have it converted to a organization account. An organization account is much more flexible for business use. This can be done according to this link https://developer.apple.com/support/enrollment/
I have basically repeated what is in the above link, if I have gotten anything wrong, someone fell free to correct me.
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