How does the Fidelius Charm work?

[Meg Rose]

No: HPFGUIDX 23211

I believe that the Charm may only work for certain people..>  Does 
that make sense?  Well, what I mean is that Only Voldy and the death 
eaters would have a problem finding James/Lily while their friends 
could easily access thier hiding place?  I don't know...  I'm just 
despretly trying to (I don't know how to word this...)  Disprove a 
potential mistake...