To get the invite there is only one criteria: you must be a WF member with a post count greater than 10, and follow these two steps:
1. E-mail me at stresstipsinfo AT gmail Dot com to get the invite with the subject "Invite me" and your WF name in the body. This is to verify you meet the above criteria for step 2,
2. Post a simple message here along the lines of e-mail sent.
That's it. I'll check your WF member name in the body against this thread to verify your active member and not somebody joining the forum just for a G+ invite, and as long as your post is here and your post count 10 or higher you get the invite. But do not PM me as you'll only fill up my WF inbox. When the invites are gone, I'll update the thread to reflect that.