Hey there, and welcome! To add an authorized user to your account who can make account changes or decisions in a retail location, you'll just need to Contact Us. Just bear in mind that once this user is added to your account, they'll be able to do almost anything that you (as the Billing Responsible Party) can do, in the store or over the phone. Is there anything in particular that you wanted them to be able to accomplish?
Manage your T-Mobile account