I don't think Fredo Corleone was an important participant during Don Corleone's meeting with Virgil Sollozzo; I think Don Corleone only invited him to witness the meeting because he wanted to show him how to react while attending important venues involving the Corleone Family's businesses.
Given that Vito had already made up his mind that his answer would be no, all the other participants were only there to show family unity and strength, to learn, and to ID Sollozzo since Vito saw Sollozzo as a potential threat.
