It is permissible to say Birkat Hamazon at an individual table during a wedding and before Sheva Brachot? Is it permissible to make a mezuman or better to Bench alone?

It is permissible to leave a wedding before Birkat Hamazon( Grace) where Sheva Brachot will be recited later. One explanation given is that the obligation to participate in Sheva Brachot is incumbent - not on all those who participated in the meal - but only on those who participated in the Birkat Hamazon at the end of the meal (Shu"t Tzitz Eliezer; Shu"t Shevet Halevi). Another explanation is that since it is common practice not to stay until the end of the wedding, guests do not automatically commit themselves just by eating to staying until Sheva Brachot (Hanisuim K'hilchatam, ch.14 n.84). If you are leaving early - your best alternative is to find others who are leaving early as well so that you can say a Zimun. May there be S'machot B'Yisrael!