Hello Jelbin,
Are your users logging on using an Active Direcory account? If so, you can have the portal accept the SPNego ticket and thus have single sign-on to the portal based on the AD user.
Check out this blog as a starting point; Configuring and troubleshooting SPNego -- Part 1. It's a bit old, but it will point you in the right direction if this setup is what you might want.
Kindest regards, Wilbert