Welcome to the Community and thank you for posting.
Setting up a Duo subscription is very easy and I'll walk you through the steps. Whenever you create a group subscription, one account has to be the administrator.
Since your girlfriend is already Premium, she can be the Duo's main account. She just needs to access her Account Overview and choose the Duo subscription from the available plans.
She'll have to follow the steps to register a home address and then, invite you to the plan. Nothing will change on your account and library, you'll keep the same music.
Keep in mind that the only account that will be billed will be your girlfriend's (since she's the administrator).
In this FAQ you can find more information about the Duo subscription. And if you still have any questions, you can let us know.