To be “up for it” is to be ready to face a certain situation. How can I say it in French with the same emotion?
If the challenge referred to in this expression is collectively faced, like if the person asking “Are you up for it?” is also part of the action, these are also valid (and heavily used in casual context, by the way):
T'es dans le coup ?
T'es de la partie ?
The example “Who wants to go to the pub? Up for it?” is an excellent example where it could be used.