ATLAS FormBuilder - Adding a Membership Provider for a Security Set
Before a security set's information can be used in a form, it must first be "connected" to that form through a form component called a membership provider. This membership provider provides the security set with a value from the form's data to use as a key for dynamically selecting from the security set's categories.
Without a membership provider, a security set has no way of knowing which of its categories' members to assign to a form, and its information cannot be accessed.
To create a new security set membership provider within your form...
Step 1 - From the dashboard of your form, select Manage in the header of the "Security and Form Lifecycle" panel.

Step 2 - In the Lifecycle menu, click the Manage button next to "Membership Providers (advanced)".

Step 3 - Now on the Membership Providers page, click Add New Membership Provider in the header of the page's table.

Step 4 - A window will open, containing several editable properties of the new membership provider. Assign your membership provider a Name, and select its Source Type as "Security Set". Use the Security Set dropdown to select the security set you wish to connect.

Step 5 - Using the "Text Merge Options" menu on the right side of the editing window, select one or more merge fields to comprise the membership provider's Selection Key.

When the Selection Key matches with one of the chosen security set's Category Keys, the membership provider will populate with the member information of the keyed security set category. For this reason, it is only recommended that a Selection Key be used if it can match with at least one Category Key of the target security set.
In many cases, it may be a good idea to create a question that can only output answer values matching a security set's Category Keys, for the purpose of using its text merge as a Selection Key.
Step 6 - Click the Save button when you're finished making changes.

Your new membership provider will be created, and ready for use in text merge and permission settings across the form!

For more instructional documentation on ATLAS FormBuilder, visit FormBuilder - Homepage