To add a custom role just go ‘Manage users’ - ‘Custom roles’ and click the ‘Add custom role’ button. This takes you to the custom role page. First, enter the role's name in the ‘Name’ field, then tick the privileges you wish to include for the role. Once you are done with that save the page. 

The custom roles the user creates will be listed on the ‘Custom roles’ page. The created custom role will be also visible for other local admins in the ‘Roles & Privileges’ step under ‘User roles’ field.