Matt's People! STEVE would have gone if the people he knows already are there. He'll be happy to make introductions, or re-introductions, or just shaking hands and being friendly.
SHITTY went with his crew. There'll be some waterworks, even if he doesn't really know them. Lots of support from this guy.
KRATOS did not attend, sorry <3
BRIGITTE was there in a pretty dress and very proud of the couple! She will have made them a little robot named Jeepers, whose only function is really to just sit around and answer questions and take notes if asked (recordings).
ARTHUR is there, looking sharp in a suit and cheer the couple on.
FJORD would've attended in his fancy suit along with the rest of the Nein and got them a ship in a bottle as a gift.
ZUKO will be there in a Toph-made mask and will spend time with the Avatar people and will actually probably enjoy himself a bit.
GLENN will be there sans Bagels because he's never seen an Outlander wedding and is honored to be invited. There will be tears and celebrations.