If your account had Advertisers before the concept of Agencies was introduced, it will be necessary to associate these existing Advertisers with Agencies.
If you click Edit Agency when there are Advertisers that have not yet been associated with any Agency within your account, Megaphone will automatically display a dialog that helps you create those associations.
After creating a new Agency, you can click Edit Agency to check whether there are any unassociated Advertisers, or to add new Advertisers.
When you see this dialog, select all Advertisers that should be associated with this Agency by clicking their + icons. Then click Assign to Agency.
About Advertisers that work with multiple Agencies
To reflect real business relationships, companies may be added as Advertisers separately to multiple Agencies. You should create a distinct Advertiser with the same name under any additional Agencies with whom they work.