How can I find out when a guide adds a new time for a tour I am interested in?

Go to the tour page of any tour you're interested in (you can always find them here) and tap the "Turn notifications on" button on that page. You will then get an email alert whenever the guide adds a new time for that tour so that you can book your slot right away. 
You can turn notifications off for that tour just as quickly at any time by tapping that same button on the tour page.