I have custom object which is private where users share records with other users manually. I want to send email notification to the shared user whenever a record is shared to him. How can I achieve this ?
You can query back the share records .share records will have userId with whom its shared and also the record Id so all you will need to do is use messaging.email to send to the respective users
Here is the document link you can use to get started .
Above on how to use outbound email functionality.