After you are logged in, you will be able to access all current open jobs which can be accessed by clicking "Jobs" on the left side menu.
Each of these job listings will have a "Refer" button. Click this button to refer anyone whom you think is a great fit for the job.
This will take you to a page where you can fill in the details and refer your contact against this job.
This will send out an email inviting them to apply for this job that you just referred.
You will be able to track all your referrals under "Referrals" -> "Track referred jobs"
Enjoy helping others to find an opportunity :)