To add a new agreement for an existing site you can simply select the site and the create the agreement option. Alternatively you can use the New Business Wizard from the New Business tab, picking the correct customer and site to add the agreement to.
If you are creating multiple agreements for a site that is not yet live be sure to se the Is Active flag to equal 'All' on the search panel.
Below details the stages involved:
This will now create a new business agreement for the site and select it for you ready to add items.