You can add a model to your Studio account if the model follows these simple steps:
1. She needs to log into her SkyPrivate account
2. She needs to go to Settings ⇒ Account and click on the "Add me under studio" tab
3. There, she needs to write your Studio's username and click Save.
Important:
Make sure you give her the correct SkyPrivate username of your Studio to avoid getting the account linked under the wrong Studio account.