Hide the "Share with" field from the Portal. This is not necessarily Mandatory for users. It sends annoying emails to all organization member. Please add an option to hide it as this is confusing to the users. 

Workaround

Here are some of the workaround that will help reduce the impact

• Disable notification for “Organization added” event from Project Settings > Customer notifications

• Disable automatic sharing of request with organization unless the customer manually selects an org at https://<YourSite>.atlassian.net/jira/settings/products/jira-service-management-configuration 

• • Set "Should new requests automatically be shared with a customer's organization?" to "No, don't share email requests with the customer's organization. Requests sent from the portal will not be shared unless the customer selects otherwise" option.

• Make sure to set the customer share permissions as per your requirement