iOS prompting for AT&T employees credentials on setup
This sounds like Activation Lock (Find My iPhone). That's a bummer, they shouldn't have signed in like that. If this is a new device I'd take it back and get another one, personally. Don't have them setup your phone afterwards.

If you don't want to return it, you'll need that Apple ID account name and password in order to complete setting it up and then signing out of iCloud to turn off Find My iPhone.