Aren't they technically colonies of hundreds of tiny organisms?
The term "jellies," or, "jellyfish," actually applies to a number of separate classifications of organisms, so it gets a bit complicated.
I don't believe that the hydrozoans and scyphozoans (typical jellies) are colonies of organisms. They're individual organisms. However, there are some
related organisms (like the Portugese Man'o'War) that ARE colonies of specialized individual organisms.
Given the broad range of organisms the generic name can apply to, that's a bit of a complicated question. The best I can tell you is that in the stricter sense of the term "jellyfish" (i.e., hydrozoans and scyphozoans), no. In the more general sense: some are.
EDIT: Okay, I had to look a couple of things up. The colonial organisms of this type are known as Siphonophores, which exist in an order that is a subset of the class Hydrozoa. The term "jellyfish" CAN be applied to ALL scyphozoans, but NOT ALL hydrozoans. So it's still a bit of a gray area, but I don't think the colonial hydrozoans can be considered jellyfish.