Though the group does not have an official lead singer, they usually hire guest musicians to contribute vocals and/or guitar parts to their studio recordings and live shows, including Andreas Kisser of Sepultura and Mark Osegueda of Death Angel.
Mark shortly after recruited then-Megadeth bassist David Ellefson and drummer Mike Portnoy (Dream Theater, The Winery Dogs, Sons of Apollo and Flying Colors) to perform on Motörhead's Motörboat Cruise for their first ever live appearance as Metal Allegiance, which was described as "a celebration of heavy metal, powered by the almost tribal bond shared between the extreme music community's most revered trailblazers, armed with a list of contributors onstage (and off) that read like a Wikipedia entry on the genre itself.
The result, the eponymous Metal Allegiance, was released on September 17, 2015, and included a number of guests, including vocals by Phil Anselmo, Randy Blythe, Troy Sanders, Chuck Billy, Mark Osegueda, Cristina Scabbia, Matt Heafy, Doug Pinnick, Jamey Jasta, Chris Jericho, Tim "Ripper" Owens, Alissa White-Gluz and Steve "Zetro" Souza, guitar by Heafy, Gary Holt, Phil Demmel, Andreas Kisser, Misha Mansoor, Ben Weinman, Charlie Benante and Ron "Bumblefoot" Thal, and bass by Rex Brown.
The EP includes covers of Motörhead's "Iron Fist", Bowie's "Suffragette City" and the Eagles' "Life in the Fast Lane", with vocals provided by Troy Sanders, Mark Osegueda and Alissa White-Gluz respectively.
In addition to Osegueda and Sanders, guest vocals on this album include Trevor Strnad, John Bush, Bobby "Blitz" Ellsworth, Mark Tornillo, Johan Hegg, Max Cavalera and Floor Jansen.