At first some feedback:
This is a very useful plugin and I think p2 alone is pretty useless without it. I'm currently just fighting with the e-mail firewall but it works with free mail adresses like hotmail out of the box. (contactform7 mails reach also the business addresses)
The whole p2 with E-Mail thing could completely replace the emails in our company if you could add one feature: All the people mentioned in the post should get an email for every new comment. This would be the equivalent of a project like a new website where a subteam of 5 people are involved, who should all get notified on every new progress without the need to mention them each time.
My question on the feature request:
I'm a developer myself and I think I could add this feature (at least if I enable it for everyone (not chooseable via dropdown). In this case I wouldn't be able to update your plugin anymore :-(. If you don't want to write the feature yourself (would be great if you want to add this feature yourself) I could also send you the codesnippet when I'm done and you could at least implement it in the next version. Are you with me?