I've stayed at the Newark Airport Hilton and the Newark Penn Station Hilton.
The Newark Penn Station Hilton is much easier to get in and out of Manhattan since it is right on top of the NJ Transit line. I think the tickets were $5 each way to NY Penn Station.
You can get in and out of Manhattan from any Newark hotel with an airport shuttle, but you waste a ton of time waiting on your airport shuttle to pick you up from the airport and also pay double the regular NJ transit fare to originate from the airport. This is one of the main reasons I like the Hilton NJ Penn Station
I think the Hilton Newark Penn Station is 35k points per night and most of the Manhattan hotels are 50k.
Your big problem will be 5 people, not many standard hotel room accommodate 5 (especially in Manhattan). They do now have premium Hilton rewards where you can book a Suite with points, but they typically aren't a good value per point.