I think it is clear http://tapastic.com/publisher/FAQ
Creator must be:
Eighteen (18) years of age or older.
In addition, eligibility is determined by, but not limited to:
Historical track record of creating and publishing Episodes on Schedule.
Commitment to maintaining a Schedule in the foreseeable future.
Adequate performance (e.g. subscribers, page views, likes, comments).
Tapastic Staff closely monitors all current webcomics and visual stories on Tapastic. We also receive feedback from our creators and readers. Based on various inputs, we send out invitations on a weekly basis.
We basically have to work for it!