You'll probably need to use PHP to do this efficiently as you'd want to batch all the emails together into a BCC so your site only has to send 1 email to reach all the recipients. As for acting on activity create you'd use the activity_onAfterCreateStreamActivity trigger shown below (var3 being the activity entry).

Example as follows.

Global
Triggers: activity_onAfterCreateStreamActivity
Type: Code
User: Automatic
Access: USERGROUP_A_HERE
Action
Method: PHP
Code:
You can use Joomla and CB API in your code action or just native PHP, but you'd basically need to query for the list of users you want to email and set their email addresses into the BCC.

We do not provide coding assistance outside the business membership, sorry. This is beyond a simple example and is a specific usecase.

The only non-coding option would be to chain auto actions, but that will result in 1 email sent per recipient instead of a single email sent with BCC, which is fine if we're talking maybe 10 or so recipients, but if it's a lot it really should be done as BCC. I can provide an example of how to do this, but the warning is it isn't sustainable as a growing site will slam your mail server if it has to send out to a lot of recipients.

As a side note I'll see if I can quickly improve the Email action to support custom query for recipients. Then all you'd need is a simple database query for the recipients.

Ok, latest CB Auto Actions build release now supports Query and Code recipients. This makes it a bit easier, but you'll still need some custom coding. It however is simple enough that I can provide an example.

Global
Triggers: activity_onAfterCreateStreamActivity
Type: Email
User: Automatic
Access: USERGROUP_A_HERE
Action
To: none
Subject: EMAIL_SUBJECT_HERE
Body: EMAIL_BODY_HERE
BCC: query
BCC Query:
Note be sure you set "To" to literally the word none as it suppresses the To address entirely for a BCC only email. Same with "BCC" being literally set to the word query. Both are noted in parameter descriptions. Replace USERGROUP_ID_HERE with the id of the usergroup you want to check for. Adjust the query as needed if you need to check for more than 1 usergroup. This should then email all those users as BCC.