Hi tuxduino thanks a lot for your answer 
yep that was the same thing i was thinking while writing the code (aaarghhhh XD)
So you suggest to use a bidimensional array like "listuser[number of user][user code]"
I save the users list on a txt file in to the SD memory and delete from the arduino flash memory. The when the program starts again it's going to read the txt file with all the user, but is the arduino capable to recognize and rewrite or delete users like before when all the users were written in the flash memory?? That is my biggest question. Then before the arduino turn off i have to save the txt again in to the SD with all the changes made.
Thanks again!