After you click Send or Add a Share outside of the organization? dialog box will appear.
Click CONTINUE to add the member to the Shared Drive.
Click CANCEL to not add the member to the Shared Drive.
Adding members without existing Google Accounts
When you try to add someone to a Shared Drive who does not have a Google account, a pop up at the bottom of the Add new members dialogue box will appear for 5 seconds, with a message stating Sharing to email addresses without a Google Account is not yet supported.
This message indicates that the email address being added does not have a Google Account associated with it. Any email address can have an associated Google Account. Having a Google Account does not require a Google Email (Gmail) address.
The email owner will need to create a Google Account so they can be added to the Shared Drive.