A Co-Author cannot create a brand new form. Only an author can. Once the form has been created by the author, he/she can assign others the role of co-author. Co-authors can then modify a form, modify the data in the Responses view and add other users to the form.
A Co-Author cannot delete a form though, only the author.
Please let us know if you have any questions!
We now also provide Team plans: https://www.acrobat.com/formscentral/en/pricing/team-pricing.html
You can buy mutliple seats of FormsCentral at a reduced price. This allows multiple people in your group to create forms.