Is Uber easy to use in Munich and Vienna for a group of 6? Can you use a charge card and/or Euros?
Perhaps you haven't used Uber before?
Uber is always paid through the CC you have attached to the App.
An Uber able to transport 6 people seems unlikely.
To use Uber you download the app on your phone. A credit card is required on the account and no transaction occurs between you and the driver.
An Uber XL fits five riders, so to accommodate 6 people you would need to order two Ubers.
To use Uber you will need to be connected to the internet, either by data roaming or WiFi.
Public transport in Vienna is so cheap, easy and clean that I cannot even imagine a need to use Uber, to be honest.