personally I dont think we'll ever take friends. I think if they have other siblings to go on the rides and hang out with that should be enough. For us, its a family trip and bringing friends would not only add a huge expense, but also take away from our family time. Just my opinion.
Last year we went with DH's entire family so it was 6 kids in total and this year we are going with my entire family and there will be 6 kids in total going this year too.