You cannot par-take in this place until you suspend your journey to the point where you can leave it at any time... That's the main point to note.
It costs absolutely nothing to try, and you suffer no penalties for losing any of them. Because we're just nice like that.
When you first start-up on this thing, you'll be given a Rank (E Rank to be more precise...). That signals what stages you can partake in. Trying to go for a Rank of a higher status than your current rank will simply force you to change your decision. Each time you complete a stage, your rank has the possibility of going up:
E - (2 Clears to reach D Rank)
D - (5 Clears to reach C Rank)
C - (10 Clears to reach B Rank)
B - (16 Clears to reach A Rank)
A - (22 Clears to reach AA Rank)
AA - (28 Clears to reach S Rank)
S - (38 Clears to reach SS Rank)
SS - (49 Clears to reach X Rank)
X - (50 Clears to reach XX Rank)
XX - 51 Clears
You may repeat any stage you have already completed, but it won't count as a 'Clear'. Only the stages which you have yet to complete can be completed to get a 'Clear' and a potential rank upgrade!
As for prizes. Well....
E - N/A
D - A choice of 10 Revive Keg's, 2 Revive's, or 1 Max Revive.
C - Promotion Card
B - Either an Energizer or Hungerizer
A - Evolution Item of your choosing
AA - ExP Share
S - Any Non-Evolved, Non-Legendary Pokemon, (Level 1, Gender of your choosing) of your choice.
SS - A Choice Band, a Choice Scarf and a Choice Specs
X - Choose two Pokemon in your team, those two Pokemoneither now have 100 Happiness, either negative or positive, on your choosing. Or has a colored Happiness effect.
XX - Master Ball
As an added incentive to get people to have a go, the first member to complete any event will recieve a hidden bonus. Which could be an item, money, or something else. You won't know until you beat it first!