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:
Code:
SELECT DISTINCT u.`email`
FROM `#__users` AS u
INNER JOIN `#__user_usergroup_map` AS g
ON g.`user_id` = u.`id`
WHERE g.`group_id` = USERGROUP_ID_HERE
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.