Don't do it. That is the best advise I could think of. It will not last. There are some very creative ways to cover an old firplace done in red brick. But in any case red brick will need to be cleaned first, that means using a soot remover, and doing a mild acid etch, both procedures will increase the bond of whatever cementitious material to the brick surface. Grout is not made for this purpose and will eventually crack or flake of, and they really can't stand the heat, basicallly you want something that will hold the pebbles to the brick for a long time, I would use a high latex modified colored mortar w/sand, sold at you local masonary yard that will last longer than grout, but you must prep the brick first.
Michael C. Malizia
The Grout Dye Comany