On the left sidebar menu click on:
On the top right click '+Add Manager"
Once you have saved their contact information and clicked 'Save' their permission settings will appear so you can edit or modify what they will have access to.
Your new manager will receive an SMS notification letting them know they have been invited to help manage the account. They will then be able to toggle to the manager view on the app, as well as login from a desktop to login to the account.