One of the features that is sorely missed in the Salesforce Lightning Experience is the ability to manually share records with another user. The feature was available out-of-box in Salesforce Classic as shown in the image below.

As a workaround, Salesforce Labs came up with a lightning component on AppExchange called ‘Lightning Sharing’ that will allow your users to manually share records in Lightning Experience.
Here is step-by-step guide showing how to install, configure and use Lightning Sharing to manually share records in Lightning Experience – all in just under 20-30 minutes.
And now the good news is that with Salesforce Summer ’21 release, the feature is expected to be available out of the box. That means you will no longer need to install the component from AppExchange. Here is the link to the Idea that says that the feature is now planned to be released in Summer 21 – https://trailblazer.salesforce.com/ideaView?id=0873A000000LmluQAC
References & Useful URLs
- Lightning Sharing Component on AppExchange – https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3A00000EFp0ZUAT