Filters will only filter by Asset and/or by User IDs. If those users posts have a unique asset then you can use that asset in your filter. If they don't have a unique asset then the only other way to accomplish that is a Query or Code field that returns a comma separated list of user ids that you can substitute into the User filter.

The best way to do this is to give each usergroup their own stream via the asset parameter so their activity is properly separated. For example lets assume you've a CB field to select what type of user they are. Lets call it cb_profiletype and lets assume its values are "bronze", "silver", and "gold". Now you'd edit the Activity tab in CB > Tab Management and set the Asset parameter to "profile.[user_id].[cb_profiletype]". What this would do is make sure all future posts by the user are linked to that specific asset. So if I'm a bronze user and I post on my activity stream then the activity will have an asset of "profile.42.bronze" for example. This will now allow you to filter by profile type since you could add a filter of "profile.%.bronze" to show all activity posts for bronze users.

There is no way to filter a stream directly by usergroup. The only way you'd be able to do that is as mentioned earlier by querying the database for that information and substituting that query field into a User filter, but if it returns too many users it could significantly hurt the performance of the stream.

You can't override the asset of Recent Activity. It will always be forced to "all". Instead change the Action from Recent Activity to just Activity for a fully customizable menu stream.