Cassia Holt Cassia (except she definitely didn't go for that cleavage and instead wore a green cami under it and she has her normal long, brown hair) was there as, you know, fellow Panem people to both the bride and groom. High five.
Cassia isn't super emotional in general, but she remembers when Annie and Finnick got married and it just makes her really happy that she's getting to see it again, this time in real life. She gave hugs to them both and congratulated them, but otherwise probably stayed close to Denver and Jillian, if she attended. She sought out Romy at some point, when she wasn't busy with family and wedding things. She wouldn't have gone out of her way to dance, but would have probably relented if someone asked her to.