Easiest is probably to build a stable wooden frame (2x4s) about 4 inches smaller than the granite top that the legs attach to and then use that carpet stabilizer stuff on top of the frame so the granite doesn't shift if someone leans against it. Paint the wood a color that coordinates with the granite and it will be virtually undetectable unless someone is under the table. The 4 inch difference between the granite and wood will give you a 2 inch overhang on each side, which will mask the wood.
Contact cement will attach the legs and wood to the bottom of the granite.
Hi. You can use bolts designed for stone and brick, but if the granite is not that thick, I suggest you attaching it to hardiboard then attaching the legs. The boards are pretty thin, but will give the granite a more stable base. There are some commercial glues that are quite strong, but I've not used them on steel to stone. Good luck!