You can configure the form access by going to the “Security” option in the project explorer. There you choose “Form access”. This should give you options to allow and restrict access to the forms. For a detailed step-by-step description of how to configure form access you can check the “How To”: https://world.mendix.com/display/Howto/Form+access+configuration
I already asked a similar question and Benny posted the following answer: Currently, form access is only used to constrain the visibility of the navigation items that show up in the user's menu bar. Form access cannot be used to constrain access to forms for user(s) (roles) but is only used to constrain navigation tree items.
Forms are pieces of HTML with optional XML configuration and never contain data.
Data is accessed based on the security profile associated with the current user session. The security checks to enforce this are performed on the server and never on the client.
If, in your project, you allow a user to access a form which requires data that this user is not allowed to access, it will generate a security exception. I suggest following Christopher's link on configuring form access to learn more about this.