To add a meter to an existing site, first head to your Portfolio on True. This is where you can find all your sites and meters, and manage them in one place.
From the dashboard, select the 'Portfolio' tile.
Select an existing site in your Portfolio by clicking the 'Manage' button.
You should then be able to see a breakdown of this site's consumption data, along with a list of the meter(s) associated to the site.
To add a new one to the site, select the '+' symbol above the meter list.
Then, follow the instructions to locate your chosen meter and add it to the existing site. If True is unable to find your meter, make sure you have selected the right meter type from the drop down, and have entered the meter number correctly.