I've just recently purchased Crue Fest 1, which was a concert event that featured 5 bands. I'm wondering if, under the cast section, I should be listing each individual musician (with or without the instrument they're playing), each group that participated, or both?

As northbloke alluded, credit them as they are credited on-screen if on-screen credits exist.  If they don't, well, it's basically up to you how to format the credits.  The way I did it with a recent uncredited set was that I listed them thusly (using Bill Monroe and his band as the example):

Quote:
Bill Monroe - himself (uncredited)
Kenny Baker - The Bluegrass Boys (uncredited)
James Monroe - The Bluegrass Boys (uncredited)
Skip Payne - The Bluegrass Boys (uncredited)
Rual Yarbrough - The Bluegrass Boys (uncredited)

I certainly could have chosen to list them in different, and equally correct ways, such as:

Quote:
Bill Monroe - himself; vocals, mandolin (uncredited)
Kenny Baker - The Bluegrass Boys; fiddle (uncredited)
etc.

or

Quote:
Bill Monroe - vocals, mandolin (uncredited)
Kenny Baker - fiddle (uncredited)
etc.

Though, as the feature I did (and the one you mentioned) has more than one band, I would not choose the last option, since that does not include the band name.  I really think the last option is only correct for music DVDs that only feature one band.

(edit)Oh, and as northbloke also said - be sure to include how you know who they are.  The contribution notes I typed up for that contribution were the longest ones ever, as I was documenting how I identified those members of The Bluegrass Boys.