Each logged in user whose has data saved will get their own UserInfo data block with a score, ten ints and five 50-character strings. One user's data will be different from another's only in the values saved (not including their user image and name, which are identifying information).

And how do I access one user's data or another? And when I use "store().. etc...", how do i tell Greenfoot that I want these data to be stored to the x-user, not the y-user?

You can only have each user store their own data. One user cannot store data for another user. The stored data for any user can be acquired while any user is running the scenario; but only the data for the current user can be change. This discussion thread contains a method to list the data for all users. The method came from my Private Messaging scenario.

Oh thank you very much for all this code! :) I have one last question please: what is the string returned when we use getUserName() if we never set a name? there's no method to set the user's name...

The 'getUserName' method will return the greenfoot member's name -- for you, it will return 'Vellyxenya'; for me, 'danpost'. Obviously, this piece of data cannot be altered. There is hardly ever a need to refer to a user specifically by name. This is the reason one must be loggied in to be able to have their data stored while running a scenario.