John Godley

User Permissions is a plugin that takes the simple WordPress permissions models one step further and allows you to assign permissions to specific posts that restrict users or roles from reading or editing it.
To summarize:
<ul>
<li>Restrict reading or writing of post or page according to the user’s role (i.e. prevent non-registered users reading a post)</li>
<li>Allow specific users to read or write a post or page</li>
<li>Permissions are assigned directly inside the post itself – there is no additional interface</li>
<li>Fully localized</li>
</ul>
<h3>Documentation</h3>
Full documentation can be found on the <a href="http://urbangiraffe.com/plugins/user-permissions/" rel="nofollow ugc">User Permissions Page</a> page.

User Permissions

This plugin provides the ability to configure post-specific permissions.

Rapidweb

Edit user permissions for Ninja Forms.
By default, all Ninja Forms options are only available to users with the ‘Administrator’ role. This plugin will enable you to set which elements of Ninja Forms different user roles should have access to.

User Permissions vs Permissions Editor for Ninja Forms