I've been by many sukkos where after the bal habayis (bal hasukkah?) "invites" the ushpizin they will then set a place for them (whichever one is supposed to come that night.) The "place" they set is simply leaving a chair open somewhere by the table or within the sukkah. What is the source for this minhag?
Nitei Gavriel Sukkah 37:6:10 says that some have a Minhag to place a special chair for the Ushpizin. He mentions it the name of the Chida - Avodas Hakodesh 289 - He also mentions it in the name of the Kaf HaChaim 639:8. Kaf HaChaim 639:8 mentions it from the Zohar Parshas Lech Lecha 93:1. Sefer Be'er Miriam says he saw the Chida do so.