Restrict User Roles in Add New Users Plugin

Everything I say below is going to be related to the “Add New Users” plugin.

How do I restrict the User Roles that can be added through the Add New Users plugin?

I’m allowing others that are not administrators (but a custom role that I have created) to use this plugin through messing around with Roles and permissions. I do not want them to have the ability to add new users with the capabilities that Administrators have.

I do not want anyone to be able to use the Add New Users plugin to create Administrators, Editors, Authors, Contributors, or Subscribers (any of the default WordPress User Roles). Resulting in only two user roles that I have created being options when using the plugin’s interface.

This probably will require code tweaking, but, since it is a WPMU Dev plugin, can a staff devleloper (or another coder in the community) help me out with this?