Speaking personally, without consulting my colleagues here, I don’t really see this as a forum function. Yes – it could be a useful plugin but could be tricky to put together. The data selection is the easy part. Sending bulk email can be fraught…

What I would suggest firstly – is to review all of the current WP plugins available that might do what you want. If they are any good and really written very well, they might (and really should) include the option to filter the recipient list via custom code. If you find one that allows for that then I would see no issue in us looking to write an interface plugin for selecting and filtering the users. This would be a better option from our point of view and quite possibly offer a broader and more sophisticated end result than we would be likely to choose to implement should we ever plan on such a plugin.

Well I have one I use today.
It is called Mass Mail , works great but is an abandoned project.

It first asks me for these 3 options:
Table name: My value = wp_users
Table email field: My value = user_email
Sender email: sender@modomain.com (example)

Then I can enter the following:
Subject:
Message Text:
Seconds between messages: 0.1 (default)

I used it on a previous site also.
There I wrote a SQL statement which created a new table in the DB based on the forum groups.
The problem being that this was a manual procedure which I had to repeat every time the user groups were modified. (delete table and re-create based on the SQL statement)

All this got to be a pain in the end bacause of so much manual work each time.
So a plugin that does everything for me would be nice.

We can resolve that one easily enough.

Instead of creating a TABLE for each user group – create a VIEW. In that way the data is dynamic and will always return the current data set at the time it is run.

Hmm, interesting.

And a view is something easily made in PHPmyAdmin? (or did I misunderstand that one?)
Do you have some links to the documentation? (yes, I like learning new stuff)

Take whatever the SQL statement is you have now to create your user group based table and turn that into a standard SELECT statement.

Then add to the front of that CREATE VIEW view_name AS SELECT etc…

This will create a View which is, in effect, a stored query – so it is dynamically run every time you use it. But it will act and behave like a table which itself can be queried. So when asked for the table name in your mail plugin – you just supply the view name instead. And voila – up to date data.

Just create individual VIEWs for all of the user groups – or indeed any grouping – you wish to use.

Thanks. Worked great.
Even got it working with multiple and individual groups.

good work… thanks for the update…