Reading the guest list for one’s meal:

The letter of the law:[1] Based on the above, one who invited guests and prepared for them delicacies and wrote before Shabbos the names of the guests which were invited so he not forget to call them on Shabbos, or he wrote the name of delicacies which he prepared for them, it is forbidden to read this writing on Shabbos. Even to read in ones thought without verbalizing is forbidden.

The custom today:[2] In today’s time it is accustomed that the servant calls for the meal from the writing which has the names of the meal guests written on it.

The reason it is permitted: There are those which have learned merit on this custom being that this is only accustomed to be done by feasts of a Mitzvah, and for the need of a Mitzvah there is not to be decreed [against reading non-document related material] due to that one may come to read laymen documents, being that the prohibition against reading laymen documents , is itself only because of “Mimtzo Cheftzecha”, and the prohibition of Mimtzo Cheftzecha does not apply by the matter of  a Mitzvah. As matters of heaven which are forbidden to be done on Shabbos are permitted to be spoken of, and no decree was made by heavenly matters that one may come to also permit mundane matters. Thus certainly there is not to decree by a Mitzvah matter against permitted matters due to that one may come to read laymen documents, and will transgress the prohibition of Mimtzo Cheftzecha.

Furthermore, it should not be prohibited here due to a decree that one may come to erase [a name from the list], as this [decree] is only relevant to decree against the host himself, which has the power to erase and diminish the amount of guests. However the servant has no power to remove [a guest] and thus there is no reason to forbid him [from reading the guest list] if not for that he may come to read laymen documents if it is not a meal of a Mitzvah[3].

([Thus] the Sages only prohibited the host himself from counting the guests from the writing due to a decree that he may come to erase [a guest]. In this decree they decreed even in a scenario involving hosting guests [of another city], despite it being a great Mitzvah. [However] the prohibition [against reading the list of guests] engraved on the board and pad due to a decree that one may come to read layman documents was only applied regarding guests which are not from another city, such as he invited local friends of his city for a friendly feast which is not a feast of a Mitzvah. Look in chapter 333 [Halacha 6[4]])

Summary:

It is forbidden for the host to read the guest list or to read the menu, even if it is a Mitzvah related feast, due to worry that the host may come to erase a guest name or a delicacy from the list. It is however accustomed to allow the servant to read the list by a Mitzvah related feast, as by a Mitzvah the Sages did not uphold their decree of “Mimtzo Cheftzecha”.