My son Jesse was pretty introverted, and even played alone on the playground until this year. He's socially experimenting with seeing his friends away from school, and even has a little girlfriend that he plays with on the playground now, along with a young man he's been friends with since he started the preschool program. I'm sure it will come. Just stay encouraged and engage them as much as possible in conversation. I almost cry thinking about what it must have been like to be Jesse a couple of years back. When he was 4, he could get 2 different 3-word utterances together, but it wasn't really speech. Talk about frustrating! 2 years of speech therapy, and now he won't stop talking!
If you have autistic children around you, I encourage you to work at it, and engage them. The best practice they get is with the closest family. They DO get embarrased when they know they are doing it wrong, but be patient, and hang in there.