Buying five individual single tickets, 25-30 euro each (full fare)

You can travel with your debit or credit card (https://www.ovpay.nl/en voor instructions) Check in at the post at Schiphol, check out at Maastricht. Do this with the carrier you are traveling with. If you transfer to another carrier, you must check out and check in again. You pay full rate.

All travelers must travel with their own debit or credit card. A ticket from a machine is €1 more expensive.

A ticket for the group (with a discount) is not valid during rush hour. Rush hour is form 06:30 AM until 09:00 AM and 4:00 PM until 6:30 PM. Valid all day on Saturday, Sunday and Dutch official holidays (not King's Day).

Better to take a taxi. Prrice is ridiculous.

I doubt you'd be able to have a taxi bring you from Schiphol to Maastricht for € 150. So the train is likely to be cheaper still.

When booking in advance you could take a Flixbus from Amsterdam Sloterdijk to Maastricht for about 10 euro per person (I got that fare when looking just one week in the future). Then you'll only require a relatively cheap train ticket from Schiphol to Amsterdam Sloterdijk.