If I am comprehending this correctly, you create new providers as users with the role of subscriber. Then when go to create that provider under the plugin settings, it creates a drop down with all users of your site with the role of subscriber to select from. If you have 300 subscribers to your website, they would all be available as a selection to become providers even though 295 of them may just be subscribers to your blog.
Rather than intermingling the two, would it not be better to have the plugin create its own role for providers labeled provider, so when you went to create that user to become a provider you would create the user with the role of provider instead of subscriber thus giving you a selection from the drop down menu in the add provider plugin settings of only providers and not everyone that is a subscriber of your site?
I guess you could install another plugin to create your own roles - name the role provider & give it the same permissions as subscriber but adding it to the plugin would be a nice touch and keep it clean.
If I am missing something let me know! just a suggestion