To add a new Landlord, you need to go to People on the left-hand navigation, select Landlords from the drop-down menu and click the Add new Landlord button at the top of the page:

You can then fill in various fields of information about this Landlord, however, at least one of the following fields must be to complete for a Landlord to be created; Company name, Email address or First/Surname. Once you have entered the desired information, scroll down to the bottom of the page and click Save:

To then link this Landlord to a Building, go to Properties on the left-hand navigation, select Buildings from the drop-down menu and select the desired building. 

Once in the Building profile, select the Landlord tab. Start typing the name of the newly created Landlord, select them from the drop-down menu and click Save: