If sheva brachot occur at a seudah shlishit which finishes after sunset, is one allowed to drink the wine even though it is prior to making havdala? If yes, who specifically would be allowed to drink it? E.g. what about someone who was not the leader or one of the newlyweds, but just wants some for a segula?
Sh'miras Shabas K'hilchasah 59:17 reads as follows. The translation is my own and slightly free, and you should not rely on it; in any event, as always, CYLOR with any practical questions.
He notes in footnote 65 that Rabbi Moshe Feinstein is quoted by Rabbi Efrayim Greenblatt as having said that the bride and bridegroom should drink from the cups but no one else. [The way the SSK quotes him, it sounds like he means this for the latter case (that s'uda sh'lishis extends past tzes hakochavim), but I haven't checked the RE.]
See also footnote 66.
(That's it for the SSK, but note that I seem to recall hearing from Rabbi Yisrael Reisman (Brooklyn, New York) the idea that the bridegroom lead birkas hamazon and drink immediately afterward. I don't recall whether that was his advice or something he had seen or what: I merely recall his mentioning the idea. It would seem to fit in with the discussion above: according to Rav Moshe, apparently, the leader of birkas hamazon shouldn't drink unless he's the bridegroom, whereas according to some others the leader can drink anyway, so this removes some doubt as to what to do.)