Hi Pavan
if you look at the definition of Type ID - You will not find B in there. G is the option to send to users......Pickup any workflow email step and select to send to WORKFLOW Initiator (that is the user ID with leading US), the mail step will use Type ID 'G' by default (your task also has the same BOand Method as in Standard email step).
So use type ID 'G' without apprehension
Now, using TRIP.EMPLOYEE.USER.NameWithLeadingUS does not needs any extra thing from your side, it is already available in your container.
In the binding editor (from Workflow to the email task) just expand TRIP object, then Expand EMPLOYEE Object, then USER Object , then drag attribute NAMEWithLeadingUS to the left of the editor which maps to Addressstring in the task. (remember to switch on the technical names in the binding editor to see these actual names instead of their data element description)
Regards,
Modak