Click on your name, Click on Settings, there are three menu items at the top of the page Account Notifications Permissions, Click on Permissions. Relationship Viewing is the first item.
There are two ways that I know of:
One is through your "Settings", under the "Permissions" 'Tab', in 'FamilySearch'.
The other is when you select the "Contact Name" of a User/Patron who has contributed details to a record.
I hope this helps.