I don't know it well enough, it's not one of the DT albums I am very familiar with. But reading wiki I'm now thinking they asked everyone to be on it.
Portnoy described the ninth part of the Suite, “Restitution,” as being “all about making amends to people you’ve harmed.” When writing it, he did not want to write about his own personal life as it would have made the song “too specific” to himself, wanting to make the lyrics more general. He instead asked friends of the band to contribute spoken word apologies. Steve Hogarth, Steven Wilson, Jon Anderson, Steve Vai, Joe Satriani, Mikael Åkerfeldt, Corey Taylor, Daniel Gildenlöw, Neal Morse, David Ellefson, and Chris Jericho all contributed apologies which featured in the final track. Portnoy was disappointed that Dave Mustaine, Geoff Tate, Bruce Dickinson and James Hetfield declined his invitation, but was pleased with the final list of contributors.