I would add them to the secondary in any case. It's the place to add things like fruit, so you don't lose all the flavors in a boil or vigorous primary. I would add at the same rates one would for say cherries - maybe from 1 to 3 pounds, depending on how strong you want it the flavor.
Thing is, there's something of a decision point here. I doubt much will happen if you just throw the cranberries in there; they're designed to float on top of the water in acidic bogs without breaking apart, I bet they would just sit there and lend no flavor to the beer. You could chop them up, that might help. But what most people do with berries is freeze and thaw them first; that breaks down the cells and makes the fruit break up more readily. But I'm not sure even that would do it for cranberries.
The flavor we associate with cranberries is generally the taste of cooked cranberries; you could consider cooking them first. That will make a different flavor profile, and ensure that it will actually get into the beer. It'll also reduce infection risks. If you don't cook them, you may wish to dip them in starsan to kill any organisms living on the skins before doing anything else.