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 myself under studio" tab
3. There, she needs to write your Studio's username (the variable you use to log into your account) and click Save
Be sure you give her the correct SkyPrivate username of your Studio, in order to avoid getting under the wrong Studio account.