Looking for a Form Plugin w/ User Interaction

I am looking for a form plugin that can basically do this:

1. The user enters the information into the form.
2. The user has the option to select other users to “collaborate” on the form.
3. The added users would be emailed that someone asked them to collaborate.
4. The added users could add comments to the entry.

Maybe this comes in the form of a support ticket plugin?

Anyone accessing the website is a registered user so that isn’t an issue. Basically we want to use this as a way to process internal reporting forms, but we want to make sure that the reports are seen by anyone involved.

Also, we are a very small non-profit, so the plugin has to be free, or VERY inexpensive.

THANK YOU!