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

You can then fill in various fields of information about this Leaseholder, however, at least one of the following fields must be to complete for a Leaseholder 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 Leaseholder to a particular Unit, go to Properties on the left-hand navigation, select Units from the drop-down menu and select the desired Unit.

Once in the Unit profile, select the Leaseholders tab. Start typing the name of the newly created Leaseholder, select them from the drop-down menu and click Add to this unit

All leaseholders linked to that particular unit will show on this tab.