Help: Adding external member to Row-Level security roles

Hi All,

Here is scenario I am working with -

• I have an app published from workspace hosted in Premium.
• Dataset contained in app have Row-Level Security configured. 
• External sharing is enabled at tenent level , I am able to share app with external users. 

but I am not able to add exernal users to a Role.

Any help is appreciated. 

Thank you,

Nilesh