I am from Objective HTML and created Postmaster. This is a highly requested feature and is rather difficult to solve, since there are so many ways one can solve this problem.
In my mind there are 3 ways to do it, none of which are available out of the box in Postmaster, yet. The Postmaster API is more than capable of handling this stuff, but not without some advanced knowledge of OOP and PHP.
Create a new list in MailChimp or CampaignMonitor each time an entry is created. I have proposed a FT that would facilitate adding/removing emails from this list in the CP/Safecracker. There are already tags to subscribe/unsubscribe from the front-end, but there is current no mechanism to create the list programmatically. This is in the works, and is the ultimate solutions.
Create a mailing list using Matrix. So you would use Safecracker, or some other custom solution to add emails to a matrix field within each entry. In Postmaster 1.4 (the next major release) this will be available. I already have it built, but it's not yet released. I built this recently for somebody who has too many entries, and didn't need MailChimp or CM, so I used Mandrill to send the emails that were stored in a Matrix field. This is a new email service, it's really easy to use and quite effective, but it doesn't do the "subscribing" (just the sending).
Use the EE Mailing List module in some fashion. I have been avoiding integrating this for a while simply because it's not a very good module (programmatically speaking). It's not extendible and does a poor job of sending email. The only way I could think to use it better is use a transactional email API to send the emails, but use the Mailing List Module as the gatekeeper for the actual email addresses.
I can't advise on the other solutions, but I will say that Postmaster was built to solve these same exact problems. As of now, only the sending portion of that problem has been solved out of the box with Postmaster, but the subscription part is just as important and definitely needs addressed.
If you have more questions feel free to email email@example.com