The user adds the Sharing button on the page layout. However, it is not available on the record page.
The Sharing button is a standard Salesforce button used to grant wider access to data. One reason for this issue is that the Sharing setting for this object in the organization's sharing settings is set to Public Read/Write. For this reason, the Sharing button is not available on the record page.
To display the Sharing button:
- Navigate to Setup --> Administer --> Security Controls --> Sharing Settings.
- Set the sharing settings for the object in the organization's sharing settings to Private.