Well, once ours started school we always had all the boys or girls in the class, depending on whether it was DD1, DD2 or DS who was the birthday child. An occasional special friend of the opposite gender and according to some photos I've just been sorting out, one or two friends for each of my other children as well. My children liked the organised chaos and so did I, as it meant that the special needs children and those with the "badly behaved" reputation didn't get left out, as they did by most other parents. The parties didn't cost much when they were little - traditional games inside or down the fields at the end of our street were the order of the day. We never expected a return invitation from anyone - if other parents wanted small parties or none at all, that was entirely up to them (and sometimes a blessed relief when you start on the party circuit!).
Inviting just close friends is absolutely fine, but I would never, ever have left one child out of a whole class invitation. I don't know what gets into people.
I was gobsmacked taken aback once when the mother of a girl in my son's class knocked at our door when his party - all boys - was in full flow, and asked me if she could come in to hand out invitations to most, but not quite all of the boys (and not including DS) to her daughter's party the next week. I just suggested politely that she should give them out at school....I must have been in a good mood that day. 
