If it was a party I would have invited you. [His mumble sounds disgruntled, because of course he'd have invited Yamaguchi if it were that kind of thing. And if it wasn't a spur of the moment think he would have invited Yamaguchi over to Iwaizumi's as well.]

It was just me and Iwa-chan and he knows I headed home, he only lives down the road. [One of the many reasons he wound up as Oikawa's best friend, convenience helps a great deal with forming friendships. Oikawa brightens up which means he also gives Yamaguchi's middle a bit of a squeeze to communicate this.] Do you want to go say hi to him? He's been dying to meet you again! [More like Oikawa wants him to give his approval but that's irrelevant! The fact that he could barely stand up without wobbling has slipped his mind.]