the housing of the starter is the ground for it, which the only way the ground would be bad is if the engine wasnt grounded, which would cause lots of other driveability concerns besides no crank. If you got the starter checked and they said it's good that should mean the solenoid is good too (if they checked it properly) which means its most likely an ignition switch problem.

Go to the smaller wire on the starter and use a dmm to check for 12 volts when the key is turned to crank, if it doesn't read any voltage, then you have a problem from the key to the solenoid.

Hope that helps.
Aaron