The first method is to activate the post by email. Go to Settings > Email and Mobile > Email Posting Address. Fill the name as desired. Pass this email address to you blog authors. If you want to keep a check, and review the posts before letting them get published, then check the radio button for Save emails as draft posts.
Through Posterous Autopost Feature
The second method is to try autopost from posterous. This requires some extra effort. Login to posterous and create a group blog. In the autopost feature select your blogger blog for which you want to increase the authors. Invite the extra authors to post on posterous. You have increased the authors from 100 to 100x now.
