Eleanor is there because there's alcohol, and because she's just become used to attending social gatherings thrown for whatever reason. Solstice works! She may have recognized Nyx because she's definitely found Galahd.
Anyway, she's drinking to take the edge off, but she's not actually drunk. And she took the warning to watch what she drank seriously because she knows how she is drunk and doesn't really suspect it could have worse consequences yet.
Anyway, she's friendly! (Enough at least?) And kinda missing Tahani since this sort of thing was her jam.