Contact can have permissions to do certain actions if they're given authorization to login. As of the writing of this article, those permissions include:
To edit a contact, first navigate to the "contacts" navigation link then click "edit" beside the contact you're wanting to adjust permissions for.
On the "authentication" tab, if login is enabled for the contact you can set the permissions you want the contact to have by ticking the corresponding boxes. You can also change their login password from this page.