THC is definitely fat and alcohol soluble. It is not soluble in water. Fats wouldn't work in beer, so you'd make an alcohol extract. Say, soak a bunch of cannabis in grain alcohol for a few months and then add that to your beer. I can't imagine why you wouldn't just roll up a joint and have it with your beer instead, but hey, whatever. Eating cannabis is a tricky thing to judge dosage so be careful.
Also, this all assumes you want to make cannabis beer. Hemp, on the other hand, is not illegal. It's illegal to grow because this country is fighting a misguided idiotic war on drugs, and apparently a war on useful fiber plants. Hemp refers to the fiber-producing strains of cannabis. Hemp beer is made legally by several breweries, one of which is my local, The Humboldt Brewing Company. They use hemp seeds in the brew, which would be a flavor-only thing. They add the seeds to the mash. I've never thought of it as much more than a gimmick. Humboldt County, for those that don't know, is one of the centers of marijuana-growing activity in the country.
And yes, hops are the closest cousin to cannabis. You can graft the two plants to one another successfully. Back in the day, people tried grafting hop plants to cannabis rootstock in hopes that they could grow hops with THC and avoid detection by law enforcement. This procedure does not work. The hops have no THC in them no matter how you graft them.
So, to sum up, adding cannabis buds/leaves to your boil or mash will not extract any THC at all. "Hemp" beer usually refers to adding toasted hemp seeds to the mash as a flavor element. If you want to make stoney beer, be careful testing it out, and make an alcohol extract to add at bottling time.