membership website codeigniter

i am newbie here in CodeIgniter. i was asked to build a-dooid-like-site or a-aboutme-like-site for my local college so they can make their own identity card. i plan building this website using CodeIgniter since i heard CodeIgniter is the most easy to understand and i don't have more than 2 weeks to complete the task.

my questions is :

if the dooid build up with codeigniter, how could i make a function that allows people choosing their direct url. i mean, if i have a class "User" and there is a "show" method that need "username" as a parameter. the URL should seems like this one:

      www.Mysite.com/User/show/<username>
how could they make a their own url like this one:

      www.Mysite.com/<username>
i plan to restrict some "method" guest and it will be enabled when user login into site. i mean if i have a class "user" there is a "edit_profiles" method. guest can see the "show" method but logged in users will be able to "edit_profiles" after login. on the same class.

can you tell me how the CodeIgniter session library class concept? i confused with native session php. is there anybody that can show me some links contains session class tutorial that omegle shagle voojio stores session into databases?