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Rav Soloveitchik provides us with an answer.
This is an expression of Cherus the status as a free people that we are required to feel. Inviting people to join in a meal helps establish that. Only a free man can invite others to join him for a meal. A slave cannot do that. He does not ‘own’ a meal that he can invite anyone to. There is a Halachic principle of Ma SheKoneh Eved, Konah Rabo. What ever a slave takes possession of - automatically belongs to the master.
The Vilna Gaon adds that there is an element of Tzedaka involved here. That of Moes Chitin a Halacha mentioned in Talmud Yerushalmi. By inviting the ‘those in need’ to a meal one fulfils this Mitzvah.
With respect to the second portion of that phrase, there is a Mitzvah to count others into a single Korban Pesach. This is called a Chabura. The Rambam tells us tht even though one may shecht and eat the Korban Pesach alone - one should avoid opt for shechting and eating it with a group of neighbors or friends.
This is what the second part of the phrase makes reference to. It does not apply to the p[oor the way the first part of the invitation does. It applies to everyone as a reference to the preferable method of eating the Korban Pesach in a Chabura.
Rav Solovetchik explains that fourth question of the Mah Nishtana is better understood this way. The word Mesubin, which refers to reclining is used in the context of a Chabura. That is the way that word is used in a Braisa in Meseches Brachos (46b). There it states that one should not sit (Yaseiv) in a Chaburah of Amei Ha’aretz. So here too it is used in that way: Mesubin in a Chabura.
Updated: 3/28/10 - 6:47 PM CDT