I already have an online prescence(codeigniter) with user register and sign in and also have people. I wish to have wordpress in /blog folder and employ some features i want. however i don't what my people to sign in or register again to make use of wordpress. any ideas?

You have to edit your .htaccess file within the root:

RewriteEngine on 
RewriteRule ^$ /index.php [L] 
RewriteCond $1 !^(index\.php|resource|robots\.txt|favicon\.ico|blog) 
RewriteRule ^(.*)$ /index.php/$1 [L] 

Note the |blog entry, its to exclude the folder from rewrite. Your folder struture is going to be:

/application
/system
...
/blog
.htaccess
index.php

You should extend your wordpress / codeigniter setup to produce a user account / give permissions for either system if you wish to share functionality without re-creating accounts (through the user).

There's no pre-built way.

Have a look at creating the consumer account at php level wp_create_user()

EDIT

Have a look at http://codex.wordpress.org/XML-RPC_wp too

You need to share your user database with wordpress. You should also modify wordpress login code to create the cookie validity context aimed at your website, not only the folder that contains wordpress (this can ensure session availability throughout your site)