To add a Representative to any Sales Division, 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 Sales Divisions" field, a list of your Sales Division previously created will populate.
You can add as few or as many Sales Divisions selected for each Representative.