Do you have a budget for paid traffic? If so use Facebook advertising or solo ads from safe swap here http://coachlikes.com/safeswaps.
Otherwise, Google plus lets you circle 500 people a day. Search by keyword for people related to what you are going to promote and circle them. Also, look for the big names of people selling similar to what you are going to be selling and circle the people who are following them.
Example: Say I am doing a weight loss presentation. I would search Google Plus for Dr. Oz, then circle all of the people who have circled him. His followers. They have already told you they are interested in what you may be selling.
Then when you make a post about your webinar, tag that group you just circled. Out of 500 a day you should get enough to fill the webinar doing just this alone. Also create an event in google plus and invite them that way too.