Imagine this is the relative effectiveness of inviting people:
* 50% Endorsement. (If someone endorses the meeting, half of the people invited will come.)
* 10% Referral. (Looks like an endorsement is five times more effective than a simple referral.)
* 3% Two-step. (Get someone to answer an ad, and then send them more stuff and invite them.)
* 1.0% Cold call. (Gee, you would have to make 50 cold calls to equal one endorsement.)
* 0.5% A simple email inviting a person to your meeting.
These are relative numbers, but you get the idea.
Instead of running ads, etc., make personal contact with one, two or three people – build a relationship and invite them. That will be more effective than hundreds of emails.