This topic comes up every year on all WDW boards, and the divide is always intense. Some people get VERY UPSET about people that do this. But I'm here to say, we do it every single time. We invite a cousin over with her 2 kids so they can play with ours. We've never had an issue at any resort yet, but we haven't stayed at YC/BC. Worst case scenario, we'd register them as a guest. We get a 2BR villa and are well under the capacity.
Oh no, 3 more people spending $60 on snacks/food and leaving after a couple hours. LOL.
Caveat for us, our family works for Disney. I certainly wouldn't do this with more than a couple people, throw a party, or do it if it was busy, etc etc.
This post doesn't endorse breaking any rules, just stating there are always gray areas especially when it concerns keeping families and kids happy at a place that depends on family and kids being happy.