In Salesforce Classic - Admins are able to add a standard "sharing" button to most record page layouts - allowing admins to view all of the users who have access to a particular record. But as of now "sharing button" is still not available in Lightning Experience. Users will need to temporarily switch to Classic to view the information.
Salesforce Labs provides an AppExchange Component: Lightning Sharing that lets you add a Sharing action to any record detail page. Clicking the button takes you to an intuitive place to view, create or modify sharing. And this page allows the user (only admins and record owners) to share their record, or modify who it's currently shared with.
We're using custom components, so My Domain must be enabled and deployed.
Set sharing settings on whatever object you want manual sharing on (from public read/write to READ or PRIVATE)
Create a custom action on the object that you want to share.
- Name it Sharing
- Set it to Lightning Component
- Select LightningSharingAction for the component
- Optionally, set the height to 0px.
Add that action to the Lightning/S1 actions section on the page layout you want it to appear on. You may need to override the standard actions.
Happy Salesforce !