Maybe she had to work? Remember this party was thrown together at the last minute. Since it appears to be taking place at night, it's not a surprise that Momo, Marten and Claire are there, they work days at the library. Marigold works from home and sets her own hours apparently. At least we've never really seen her working. The others seem to work different shifts, and at least in Dale's case he works a couple of jobs. So it's more surprising that they are all that. Steve has an unusual job, and apparently just got back from Canada, so not terribly surprising he's available, but that doesn't mean Cosette is. Besides, she's really only friend-adjacent in the comic. Kind of like Penny or Raven, they only show up when they're needed for the plot. Steve bounces back and forth between tertiary character and plot backup these days himself.