The Shu”t Chasan Sofer (siman 14) discusses this very question. He adds that this is a very common shailah:כי עיניהם של עניים נשואות בסעודת נשואין ,ואז חוזרים חלילה וגלגול חוזר משלחן לשלחן ומפסיקין בין גברא לגברא לבקש על המחיה ועל הכלכלה – “Poor people wait for weddings, as they can go around in circles from table to table stopping lots of people asking for money to provide themselves with food and sustenance.”
He concludes: Even according to the Terumas HaDeshen and Magen Avraham that those dining at a chasunah are exempt from all mitzvos, nonetheless, they are not exempt from giving tzedokah to the poor. Certainly, according to the Rishonim that the exemption of oisek be’mitzvah patur min hamitzvah only applies if one can’t fulfil both mitzvos, here one can fulfill both therefore he is obligated. But even according to the Rishonim that even if one can still do both he is exempt, when it’s very easy to do both without any effort everyone agrees one must do both. If so, when dining at a chasunah seudah one doesn’t take away from the mitzvah of simchas chossan v’kallah by giving tzedokah, as he can remain seated, and the poor person will come over. Reb Yosef who said that if one is busy with returning a lost object is exempt from giving tzedokah is talking about a case when one is busy with his hands taking care of the lost object, however, by the mitzvah of simchas chosan v’kallah there is no need for one to use his hands, he can sit there and say some nice words, either Torah, or other things, and he doesn’t need his hands to be busy, therefore, one is certainly obligated to give tzedokah.
He adds: If one is playing an instrument and his hands are busy, then it’s true that such a person is in fact exempt.
