There are 2 methods to add a new Hiring Manager account.
Go to Users tab and then go to Hiring Managers. Use the "Add Hiring Manager" blue button in right upper corner:
Fill in the form and click "Add Hiring Manager at the bottom right:
You can click "Show more" to set up the Hiring Manager's phone number for SMS verification.
Go to a job and mark candidates you want to share with the Hiring Manager. Then choose the option "Share candidate on stage". Use the + button next to "Invite to stage" field and continue adding a new Hiring Manager account according to the description above.