If there were only five in the OP's party, he could get a one-way car rental from LGA to Manhattan on a Friday night from Avis or Budget for $14.00, including tax and gas. Unfortunately, none of the vehicles offered at the special rate can seat six.

Edited to add: Checking random Fridays in October, I am seeing a 7-passenger minivan available for about $26, all-in; and maybe $25 if one is willing to prepay.

There are Avis and Budget locations that are a short walk from the OP's hotel.