Grout, mortar, and waste slurries containing these products should never be poured down a drain. It is always best to wash your buckets and trowels outside using a garden hose.
Since grout sets up under water, hot water will not clear the clog, and there are no easy solutions. If you have plastic sewer lines, you may be able to break the grout up enough so that it is forced on down to a larger line. Unfortunately, if there is a lot of grout, you could just end up creating a clog further down the line. It is almost impossible to get grout out of steel pipes. You can try removing the bathtub drain trap and snaking the line. It may even be necessary to break up other lines and floor drains or even replace a few feet of line. Your best option is to call a licensed plumbing professional who can use powerful drain cleaning equipment to try to clear the line.