Question about how to use custom domains

Hello Team,
I have a question about how to use custom domains.
I'm currently working with Individual Plan, and my business partner using  Custom Sub Domain with an Enterprise Plan.

My  URL: app.box.com 
My business partner's URL: something.app.box.com
In this case, If my business partner adds me to their group and create share folder, Can I Login and Working with  [something.app.box.com]?
If not,How can I use Costom Sub Domain ?
※Do I have to contract with Enterplise Plan ?

Thank you.