That should be doable as long as your other usergroups are child groups of group A. Example as follows.

Public
- Registered
- - Usergroup A
- - - Technical Department
- - - Sales Department
- - - Service Department

Setting the privacy to Usergroup A should allow the 3 child usergroups to also see it due to inheritance.

If you're using multiple assets for activity and you want your stream to output multiple assets then add multiple assets to it. As the description states it supports a comma list of assets. So if you want it to display both servicedepartment and usergroupa activity then just add both assets to it as follows.

profile.[user_id].servicedepartment,profile.[user_id].usergroupa

You can even use IF substitutions in it to condition the asset based off profile data. Example as follows.

[cb:if cb_profiletype="servicedepartment"]profile.[user_id].servicedepartment,[/cb:if]profile.[user_id].usergroupa

The above for example only uses the servicedepartment asset if cb_profiletype is equal to servicedepartment. The usergroupa asset is always used. The first asset supplied is the asset used for storage so order is only important in that regard. This means a user with servicedepartment asset in the above examples will have their activity stored with that asset and usergroupa is strictly for display purposes.

The only reason for your post to store with an asset of profile.[my_id].usergroupa is because you changed the asset for the activity stream. At this point I don't understand what you're trying to do anymore, sorry. The usergroup structure proposed is to allow for your privacy selection usage to work since privacy selection of a usergroup respects inheritance. Now you're talking about issues with the asset so at this point I've no idea what your issue is now. Please describe exactly what you want your stream to do.