To put a partner on a 'Wait List', you perform a 'Deposit' task. On the 'Deposit Details' pop-up window is a radio button for you to select 'Wait List'. This will flag the prospective resident as on the 'wait-list'
Once the prospective resident is off the wait-list, you can then go into that same lead's tasks's Deposit Details, and then un-check the 'Wait List' box.
You can search for any leads you have flagged as on the wait-list by filtering with the 'Additional Filter's' drop-down menu on the lead search page, and selecting 'Wait List'.