Abigail, I love your question, and I love that YOU are a special person who notices these types of kids and are compassionate enough to care to include them! I am an occupational therapist who works with children with special needs and students without special needs. Sometimes I even teach the "typically developing" kids how to interact with the kids with greater challenges in things like school, baseball, play, etc.
The best advice I could give you is: although it might be scary and you might not know what to do, try your best to include them just like you would any other kid. Some kids are very sensitive and overwhelmed by others (so also give them space if it looks like they are uncomfortable!). You can always ask an adult that's working with them (sometimes they have aids with them) if you can chat with them or that you want to connect with them and is that okay? But try going up and saying hi! Introduce yourself, ask how they are doing, if they like dance, music, sports on TV, what subjects in school etc. Kids with special needs have allll different levels of needs and things they are good at. Once you get past the scary, unfamiliar, or uncomfortable part of talking with these types of kids, hey you never know you all might make a few new friends :). Let me know if you have any more questions or need further support!