Yesod VeShoresh HaAvodah writes, "Erev Succos… try to find proper guests, b'nei Torah, for Yom Tov." The importance of having guests on Succos is discussed in the Zohar. The Zohar teaches that the seven ushpizin, Avraham, Yitzchok, Yaakov, Moshe, Aharon, Yosef, and Dovid, come to our succahs to celebrate with us. When they arrive, they look to see if there are guests at the seudah. If guests are present, they remain in the succah and bless everyone. But if there are no guests, they leave. Therefore, it is important to invite guests on Succos.
The first day of Succos is the ushpizen of Avraham Avinu, who excelled in hachnasas orchim. Rebbe Eliezer of Dzikov zt’l was very diligent about having a guest for the first day of Succos, on the ushpizen of Avraham Avinu. One year, Reb Eliezer of Dzikov didn't have a guest for the first day of Succos. He asked his son (the Imrei Noam zt’l) to go out and find someone who needed a meal. His son walked around the streets until he found a homeless man and invited him into his father's succah.
The Dzikover Rebbe was thrilled with the guest. He said, "Avraham Avinu’s guests weren’t any better. Avraham cared so much about people that he was eager to bring anyone who desired a meal into his home. At least, on this day, we should emulate Avraham Avinu's ways."