To add a Representative to any Item Catalog, your account must be higher than a representative. Even if your account is an Administrator AND a Representative account, that is still higher than just a Representative.
From your main menu, find "Employees"
Within your Employees list, there is an "Actions" column, this is how to access the edit page. Click on any of these to edit the employee needed.
Click on the "Select Item Catalogs" field, a list of your Item Catalog previously created will populate.
You can add as few or as many Item Catalogs selected for each Representative.