If Chloe was limited on the number of kids she could invite to this party, maybe she chose kids who absolutely LOVED skating and were good at it. Not all kids are, nor do all kids enjoy it! Your daughter may have mentioned to Chloe some time ago that she didn't like skating and forgot. Some kids have friends they play baseball with, other friends they go skating with, and other friends they go to music classes with, because these kids are also taking lessons with them or just happen to be very good at it and enjoy it, for example.
Maybe these are kids she doesn't see often, maybe they are relatives, or friends from out of town, so this was planned in advance. Maybe she didn't even have control over who got invited, it could have been mom wanting to invite some kids who don't see Chloe often to have a chance to see her, or her wanting to see some moms she loves, and therefore, the kids are brought to the get-together.
While Chloe sees your daughter and speaks to her all the time, maybe that is not the case with these other kids. Some kids are busy with extracurricular activities, or live an hour away, so meeting up during the week or even on Friday evenings is difficult. Nothing wrong with giving other friends a chance to spend some time with her. If your daughter was so hurt by it, she could ask Chloe to go skating the weekend after, just the two of them, or with some additional friends that they both like, so she won't feel left out.