Apex sharing example | Trigger to validate if users have access to that record and to give access to the user when record is created using Apex Sharing | Salesforce Security

You want to validate that the recruiter and hiring manager listed on the job have access to the record. The following trigger grants the recruiter and hiring manager access when the job record is created. This example requires a custom object called Job, with two lookup fields associated with User records called Hiring_Manager and Recruiter. Also, the Job custom object should have two sharing reasons added called Hiring_Manager and Recruiter. This post is taken from Salesforce Developer Guide