VORTEILScard

The card costs 100 Euros and you will get 50% discount on your tickets. So you can calculate yourself if it's worth (ticket prices can be found on [www.oebb.at]).

If you want to go to prague keep in mind, that the discount will be only for the austrian part of the journey and for standad tickets. There are very cheap tickets (Sparschiene) for international travel that are much cheaper than a standard ticket (with or without Vorteilscard) - but you have to book early for these kind of tickets and they are limited.

Makimaki gave you the perfect information.

I just like to add something about the "special packages of ÖBB" to european cities (like Prague):
Hotels are quite cheap + still central (good choice). But you have to book at least 2 weeks in advance. ÖBB is not very spontaneous...

Have a nice trip!

If you are under 26 the card costs only 19,90.
On journeys within Austria you save 50 % (attention: only when buying online or at the vending machines, when you buy at the counter you only get a 45 % reduction)
When traveling to Prague you'll get a 25 % reduction for the trip between the border and Prague.

For longer journeys e.g. Vienna-Salzburg return this >26 card is worth it. But if you just stay around Vienna probably not.

attention!
if you dont cancel the vorteilscard after 12 months, you automatically get a new one that you also have to pay.

That is not true. When signing the contract you have the option to give them your bank account and hava an automatic renewal every year.
But you can also pay cash/credit card. And next year they will send you a letter, that your card is expiring and will advertise for a pronlongation. But if you don't answer/don't pay your card will simply expire.

Can one person in a party of 3 buy the Voerteilscard and then buy all tickets for all members in the party? Also, how long does it take to get the card if you are based in the USA?

No because you have your photo on the card and must have one per ticket. It takes me a couple weeks to get mine living in Vienna. I couldn't say if it is possible from the US.