Defining "allies" - A matter of perception?

[Jack99]

Thanks guys, lots of good opinions out there. Based on what you have said, I think I am gonna go with the caster choosing his allies at the moment of casting, in this case, when the bard starts his songs. It's easy, and makes sense. On top of that, it will enable my backstabbing NPC to start up the songs, go into hiding, and start slinging grease and dispel magic at his father at inopportune moments, without the other players losing the benefits of his song(s). This should make his undercover operation able to last a bit longer.