A lot of guest musicians have played with CEONEO in recordings and live:

Claes Antonsen: Additional drums on No Slack!

Jacob Andersen: Percussion on all songs on Father's Sun and No Slack!

Tom Atkinson: Cornet and trumpet on Mrs. Sure and To The Sky.

David Beckham: Organ on To the Sky, I Know You, You Know Me and Tick Tock Song 

Klaus Bendtsen: Hammond B3 on Sapient man and Summer.

Andy Binau: Add. keyboards on Worlds smallest Ant, add. guitar on I am so sad, Syd, guitar on I am So Sad, Syd and Listen to the Echoes. Drum pad on Ya-Ko-Ta-Kaii. Guitars and backing vocals on Buy, Buy, Buy.

Jeanette Binau: Live backing vocals.

Bailey Blankenship: Lead guitar on Never Enough.

Thomas Brekling: Guitars on Sad sad Story of Billy and Dave.

Christina Boelskifte: Lead and backing vocals on almost all songs on Father's Sun and No Slack! Lead and backing vocals on Hindsight.

Wayne Carter: Percussion on I Sing You Close to Me. 

Alec Chetbi: Snaps, claps and backing vocals on Buy, Buy, Buy

Liam Chetbi: Claps on Buy, Buy, Buy

Laurel Cooling: Vocals on Thirty Seven. Vocals on China Dolls.

Claire Derks: Lead vocal on To the Sky, Backing vocals on

Tommy Detamore: Pedal steel guitar on In this town.

Philip Diorio: Jingle bells on Good Ole Christmas.

Philip Diorio Jr.: Didgeridoo on I Sing You Close to Me.

Aaron Foley: Acoustic guitar on A Sapient Man.

Felix Guerra: Percussion on To the Sky, I Know You, You Know me, Sapient Man, Tick Tock, 

Anders Gustafsson: Trumpet on assorted songs on No Slack!

Ida Hedegaard: Live backing vocals.

Shelley Hicks: Lead and backing vocals on several CEONEO songs.

Marc Holland: Live percussion.

Christian Joergensen: Violin on assorted songs on No Slack!

Jimmy Johnson: Classical guitar on Never Enough.

Ditte Juul: Live backing vocals.

Amourose Langer: Backing vocals on I'm the One.

Rooney LaPlante: Guitars on I am so sad Syd.

Christian Lebek: Add. drums, bass, guitars, keyboards and percussion on In this town.

Gary Lovini: Violins on China Dolls and the Clown (unreleased)

Morten Lynggaard: Backing vocals on Jeg vil rejse jorden rundt (Charity single for Danish MS association).

Nanna Luders: Lead and backing vocals on So many people, How could we live without Lucy.

Robert (Otto) Morton: Guitars on I'm so sad Syd.

Marcus Mukherjee: Violin on Good Ole Christmas.

Michael Nelson: Lead vocals on In this Town.

Lars Papsoe: Guitars on In this Town.

Barbara Pearce: Speak on I Am So Sad, Syd. Speak on I Know You, You Know me. Claps on Buy, Buy, Buy.

Lonnie Radzinsky: Solo guitar on Common Man

Noah Rosanes: Guitar and keyboards on Jeg vil rejse jorden rundt (Charity single for Danish MS association).

Tamra Rosanes: Backing vocals on Jeg vil rejse jorden rundt (Charity single for Danish MS association).

Jens Runge: Guitars, mandolin, ukulele and banjo on assorted songs on Father's Sun, No Slack!, and on Good Ole Christmas, where he also played jews harp and glockenspiel.

Wendy Schall: Lead vocals on In this Town and So Many Brothers and Sisters

Garrett Smit: Lead vocals on In this Town.

Eric Thompson: Harmonica on Listen to the Echoes, Friend and Foe and Mrs. Sure.

Kristin Tucker: Lead vocal on In This Town.

Doug Turco: Percussion on Mrs. Sure.

Caro Teruel: Cello on China Dolls and the Clown (unreleased)

Karin Tuxen Naesby: Backing vocals on Jeg vil rejse jorden rundt (Charity single for Danish MS association).

Jarno Valsted: Blues harp on assorted songs on No Slack!

Leo Von: Backing vocals on China Dolls.

Helle Wagner: Backing vocals on Jeg vil rejse jorden rundt (Charity single for Danish MS association).

Austin Wessel: Church organ on China Dolls.

Amy Xaychaleune: Violin on Thirty Seven.

Others:

Bella, Buster, Mimi, Paris: Barks on Friend and Foe.