[development] creating a new usermanagement things

Hey all, When using drupal for a site where users can register themself for a event; what is the best way to store the data when you have different modules to manipulate them? By manipulating i mean: - Managing some fields like if the person have payed. - Managing multiple users to assign them to groups - Just show lists of users - Printing lists of each user data I'm right now looking a the module profile which is provided by drupal it self. but is see it is data stored in a separate table. Is it then the best way to create code which uses the "API" from profile? Is there anyone who has more experience with this? TIA, -- Kees