Does a properly flanked enemy only grant combat advantage to the allies doing the flanking, or to all allies?
Only the allies in flanking positions get combat advantage.
However, there is a feat that allows a ranged attack to benefit from flanking allies. It's called
There is also a feat for half-elf rogues that does exactly this called
Finally, there is
Vexing Flanker, which does exactly what you are talking about without the class/race specificity.