You'll need the agency's Account Id in order to invite them to manage your account. Contact support (or reach out to your CSM) to discuss inviting an agency to manage your account.
From the left navigation menu, select [Menu] → Settings.
On the left side under General, select Account Users.
In the upper-right corner of the screen, select Invite, then select Invite Agency.
Add the Agency Account Id and use [Toggle on] to control access to the product(s), then use drop-down menus to select a role (and associated permissions) for them. Select View Details to see all the permissions that will be given with the chosen role.
View our article on Brand Account User Roles & Permissions for help with setting up roles & permissions.
Select Submit to send the invitation to the agency.