This is going to be a take off of the CBmailing extension that the author has abandoned. You will be conveting this extension to a new one.
In CBmailing, you create mailing lists based on CB Lists. So, you create a List for "Group A Admins" and then another for "Group A Members". Then inside the CBmailing component, you create a pair using those Lists. One pair would be "Group A Admins" can email "Group A Members". When a Group A Admin goes to the component in the frontend, he sees his options of who he can send an email to (multi-select field). He chooses who to send an email to, then writes email and sends it.
HOW THIS EXTENSION IS DIFFERENT
The component I need will be based not on CB Lists but instead on Joomla Groups. So I would have a Group for Group A Admins and Group B Members etc. Also, I'd have Group B Admins and Group B Members, etc.
Right now the CBmailing extension does not have an HTML editor for the email body, so I'd need that added. Also, I'd need a new option in the backend for "Send from". That way you can set a specific email address to come from (so it matches the domain the email comes from and avoids spam filters). The "from name" would always be the person logged in, but the "from email" would be set in stone in the backend.
I will give you the code for the 1.5 CBmailing extension so you can alter it instead of write all new code.
You will need to convert to Joomla 2.5, add the HTML editor for the text area, add the new "From email" in the backend, and of course convert it to it uses Joomla Groups and Not CB lists.
I'm attaching the abandoned CBmailing extension for J1.5 as well as a screen shot of the main backend part.