s3tech.nabbas on "Need to use my site users to login as a wordpress blog user"

Hi
I am developing a website for my client, the site is running from last 2 years.
Now my client ask me to integrate wordpress blog for him, I have integrated this and created a theme for him.
But now he ask me that when a user on the site is login and visit the blog he should automatically be login on wordpress blog, I am new to wordpress so I am confuse how to do this task. I have also search on internet about the solution but found nothing helpful.
Please if someone help me solving this problem, send me the link or helping material so I can end this problem.
Thanks in advance.