How do typical ground marshaling signals differ for helicopters?
All of the signals used for marshaling fixed wing aircraft apply to rotary wing aircraft as well.

There are additional signals though related to hover maneuvers such as move left or right, hover left or right, forwards, etc.
and land (or rather cleared to land), take off into the hover, etc.

Marshaling large helicopters can be very difficult, especially in dust and snow conditions.

Source: ex. 7 Sqn Chinooks