Allow usergroup as an option wherever author is

[rgf100]

I'm trying to set up a feed of posts by new members. Activity streams can't do it, as you can only choose authors. The post feed block can't do it, for the same reason. 

Here's an example (down the page) of someone trying to set this up for a staff group, but having to resort to adding people by name, meaning manual changes will be needed when people join or leave the group.