facing ds's 5th in 3 weeks...
last year we had his favorite kids that are around the same age over, 4 of them, 3 brought younger/older siblings and we had the parents (if they wanted to stay) too. made it afternoon so i wouldn't have to feed them too much. got lucky and decorated the backyard, it just turned into a big playdate. it really was wonderful. we did have a couple games, like musical chairs, but it was a bit of a shock as we didn't count on crying "i lost" kids - but that only lasted a couple minutes, dh got the "loser" to turn off the music the next time. for food we just did pigs in a blanket, some fruit and veggies and chips, and cake of course.
are doing much of the same this year, 6 friends, this time no siblings unless he's very good friends with them too (as is the case with one friend).
i would ask who her best friends are, and invite them. keep it small, i read about inviting as many kids (roughly) as the birthday girl is old (this is also a nice excuse for not inviting a whole lot more kids). i don't know where the notion comes from that you'd have to invite the whole class - silly in my opinion.
ryan's in a class of 12 this year, and in fact none of the kids is in his class. there are 2 JK/SK splits and 4 kids are from the other class, one is a big brother and one is a girl that's being homeschooled. he's known most of them since birth.
keep in mind that the bigger you make this party, they'll only get bigger from here on... most likely. i'd keep it casual, a few friends, let the parents stay if they wish, if not, okay. also put a time for pickup on the invite. and don't stress. think of it as a big playdate. :-) have fun!