The Salesforce Classic has Sharing Button on record pages that gives you the ability to manually share the record whereas it is not available in Lightning.
There are many workaround available to add sharing button in Lightning. Let’s go through each of them:-
- A Flow Action called Set Sharing Manually that demonstrates the use of Lightning Web Components to create screen components that do useful ‘member management’ Salesforce administration. The component is based on seminal work by Shane McLaughlin.
- Read more here:- https://unofficialsf.com/adding-a-manual-sharing-button-to-records-with-lightning-flow/
- Contains a Lightning Component that lets you add a Sharing action onto any record detail page.
- Clicking the button takes you to a tabbed view of existing sharing (tab 1) or the option to create new shares to users or groups.
- Enforces sharing security–the record owner or admin can adjust sharing.
- Install it from here:- https://appexchange.salesforce.com/appxlistingdetail?listingId=a0N3A00000EFp0ZUAT
Also keep checking this idea to get latest update from Salesforce:- There’s no share button at Lightning Experience