How add user to my itunes account

What exactly are you trying / wanting to do ? An iTunes account is designed to be used by one person, only you should have access to your account

She can log into your account via Settings > iTunes & App Store on her device (redownloading an account's past purchases may tie her device to your account for 90 days : View and remove associated devices in iTunes - Apple Support)

Or if you want to share each other's purchases you could set up family sharing : Family Sharing - Apple Support

With family sharing a family member can use their account's balance or the organiser's card for their purchases (they can't used their own card) - the organiser can't use any balance on their account