Grohe shower leaks from faucet when shower head is selected
if you mean you have water coming from the handle part of the shower (this is the mixing valve), then it needs removal and repair.  could be anything wrong, but its usually the front seal on these cartridges.  this unit looks like it has the diverter in the mixing handle, so the whole unit probably needs repair.

if you mean the water comes from a tub filler spout with a diverter valve in it (the little pull or twist handle that selects shower instead of tub filler), then the tub filler probably needs servicing or replacement.  most of them are less costly than replacement parts for the same unit