How to handle user objects??

I have a simple "webshop" style application, where I ahve several classes (user, product security etc)

First I use forms validation to authenticate the user.. if he is authenticated I let the security class create an instance of User(or Admin) that I will store in a session object to use when I travel between the pages.

Now the problem:

In my admin class I ahve a method called "CreateUser" which will be called when the admin fill out a new user form and click submit...

but how will I call it? Do I have to get the admin object from sessiona nd call it like that? Or is there a better way to solve it?
Perhaps create the objects and responsibilities differnt? How is it normally solved?
Kind regards
Henrik

Is this way or parsing entire user objects in sessions a good idea? How cold it be solved instead?

kind regards
Henrik