In your Smartlist,> Click on "Add a filter" > Click on "Bookings" > and Click on "First booking".
Select the number of the reservation you want to find (here is the 1st reservation). Select the selection criterion you are interested in (here during the last week: there are more than 0 days and less than 7 days). Click on "Add".
You have just created a list of all the people who have made their first reservation during the last 7 days.