Their best bet would be to take Mears.
It is $16 or $17 PP roundtrip.
The day before they wish to go to Universal, dial "0" from their room phone (or any Disney house phone). When the Disney operator answers, ask to be transferred to Mears.
When Mears comes on the line, tell them you want to go to Universal the next day, how many people, where you are staying, and approximately what time you wish to leave your resort.
The Mears folks will give you a pickup time and place for the next morning.
Be at the location they state 15 minutes before the time they tell you.
When the bus/van arrives, pay the driver the fee in CASH and board the bus.
When you arrive at Universal, you will get a return bus voucher. You can make arrangements for a return time at that time OR you can call the number on the voucher 2 hours before you wish to leave and arrange a pickup time then.
When your appointed pickup time arrives be at the location at Universal where you were dropped off and you will be transported back to POP.
If you would like to pay for your Universal transportation with a credit card, stop by the Mears desk at MCO when you first arrive in Orlando. You will then receive transportation vouchers to use the bus.
You still need to make the actual bus reservation in the manner described above, but won't need to have cash.
Note, this is a shared service, so you may have to stop at other resorts both on and off Disney property both directions.