The cameo appearences are:
Mimiru
Silver Knight
Bear
BT
Tsukasa
Sora
A-20
Crim
Subaru
They can be found in movie 'memories' in Mutation, and traces/leftovers of their data or "ghosts" in the root towns during Outbreak. In the third game you do meet in person Mimiru, A-20, and Bear, while in Quarantine you meet Crim and in the epilogue you get[spoiler]Sora, Tsukasa, and Subaru's member adresses.[/spoiler]
I believe that's all of them.( --_--)[/b]