Tij from FranceYes the guitar solo is provided by Steve Lukather from Toto. From Toto as well is Jeff Porcaro who provides the drums. Speaking about the guitar solo by Lukather, recorded in only one take, unbeknownst to him! Basically Lukather told Richie and producer Carmichael: ok just play it so that I can warm up on it. They played it without telling Lukather they were recording it already. When Luke said, "ok I think I got it. I'm ready, let's do this", Richie told him: "it's done! what you just did is perfect!". Lukather's session was complete within 15min.
The lyrics for Lesley Gore's "It's My Party" were based on actual events relating to his daughter Judy's sweet 16 party. The teenager threw a tantrum and burst into tears when her mum and dad insisted that her grandparents had to be invited.