When my DD (17) had sleepovers, I ALWAYS made sure I had the phone numbers of all the parents before they dropped off their kids. They were also given a pickup time on the invitation.
If someone didn't come within, say, 30-45 minutes of the pickup time to claim their child, I called the parent(s). You have a life/home too and there's no reason why that child should still be at your house after everyone else has been picked up!