Nostalgia People You Forgot Guest Starred on It's Always Sunny In Philadelphia  

Episode: "The Gang Saves The Day" 
Role: Himself

The gang finds themselves stuck in a convenience store as it's robbed, so we see each of them imagine scenarios in their head. During Dee's daydream, among other things, she sees herself married to the singer, songwriter, (and in this case, actor) Josh Groban.

Episode: "Dennis Reynolds: An Erotic Life"
Role: Himself

Dennis winds up in a rehab facility, which is where we see the comedian playing an exaggerated version of himself. It’s worth noting that Rob Thomas from Matchbox 20 is there as well, interacting with Dennis and Sinbad.

Episode: "Charlie and Dee Find Love"
Role: Ruby Taft

The actress who starred in San Andreas and on True Detective’s first season appeared on It’s Always Sunny In Philadelphia as Ruby, a woman from a wealthy family whom Charlie is smitten with. She goes on a double date with Charlie, alongside her brother and Dee at a fancy restaurant, where Dee and Charlie struggle to pretend to be classy. 

Episode: "Charlie Got Molested"
Role: Chris Murray

When the gang discovers that Charlie’s former elementary school teacher is on trial for child molestation, they try to get Charlie to admit that he was sexually abused by him. Haskins, who famously played Mr. Belding on Saved By the Bell, was that teacher, Chris Murray, who it turns out was actually being set up.