Client account permissions split into two categories: those relating to their own user account settings, and those relating to managing and viewing the data. For information on the former, see the Editing Accounts page - especially the comments on the Settings tab.
Form Tools 2 lets you specify precisely what form information can be viewed and edited, down to each and every form field. We tried to balance functionality with usability to allow for this, so please let us know if you feel certain functionality is missing, or if the user interface is unclear.
In order for a client to view a form, they need to be both assigned to the form, and to one or more Views within the form. Assigning the client to a form is merely a generic "blanket" setting, it doesn't provide any control over the details: that's handled by the Views. Views let you customize subsets of the data - both in terms of what fields are displayed and in what order, and in terms of which results appear in the View. For more information, see the Views section. Client permissions are fundamentally about Views; once you learn how to create Views, changing permissions for a particular client account is as simple as assigning a client to the View.