I agree with all or a handful.

An interesting aside to think about re: meanness
I remember a really mean girl at kinder. A real bully, pushing kids around, not letting certain kids sit next to her, teasing, controlling equipment. I remember being quite scared of her and in hindsight I was also most likely enjoying the camaraderie of being 'in the group' of others who were being mean back to her by excluding her.

When I told mum about her, my mother suggested that we invite her over for a play and I distinctly remember my 4 year old self being mortified as I didn't like her. Mum's take on it was that most often at that age, a child who is having trouble socially and does not have the tools to fit in to the group, desperately wants to but uses force and 'bullying' to get her way. We did have the girl over, she still scared me, but, the massive difference was that she was so nice to me from then on at kinder, sticking up for me in groups, saving me seats, looking out for me. She ended up getting the friend she desperately needed but didn't know how to make and I learnt a really valuable lesson about looking beyond immediate behaviour and giving people a chance. (I may not have completely understood the lesson then but mum talked about it with me over the years and it did stick with me)

With that in mind, I made a real effort to get to know the boy who was 'picking' on my DD last year and discovered that he was really just quite smitten with her and tended to crowd her and want to play with her all the time. He was quite young and English was a second language for him and he just did not have any idea how to make friends with my DD. Knowing this helped me to teach DD to tolerate this boys overtures, be more generous with him while still acknowledging that she had a right not to play with him if she didn't want to, and eventually work out a way that they could get along in a way that kept them both happy.

BTW, I am not saying you necessarily need to do the same thing with your son and the meanies, and a birthday party may not be the time to try to change the social dynamics at childcare but I think leaving a few kids out may cause more issues than might be necessary.