Dances With Balrog is absent from the Alliance meeting.

Correct me if I'm mistaken here, but I would say the disparity in the attendance is due to how the big three are the leaders of each group (Athena Pierce, Cygnus and Claudine) while some of the others who don't have obvious connections are there as audience. Consider the Resistance as an example, the quest dialogue you get leading up to the Alliance implies that Brighton, Belle and Checky all went there along with Claudine as they still do not trust Cygnus fully and thus must see the events unfolding themselves, Lady Syl is there as she's also suspicious of this, Neinheart and Mikhail are present as the former is Cygnus' advisor and the latter is her bodyguard more or less while Lilin may be there for tworeasons: 1) She has connections with Aran and knows about the Heroes which would make her a suitable representative for them given that Lilin is one of the options for invitation if you're a Knight of Cygnus or 2) Well, she is Neinheart's sister and an important figure considering what was mentioned in #1.

So there you go. If at anything I'm more curious about why Mercedes isn't there seeing that one of her quests was pivotal towards Cygnus' decision to form the Alliance and she wouldn't have much overlap with Lilin given that she could represent her own people, frozen as most of them were at that time.