My initial reaction was that this wasn’t possible, that he had to be a member of the groups to give them access2. However, before I could send that reply, he sent a follow-up explaining he had found out how to to do it.
I’d actually thought about this problem a long time ago and implemented a solution. In the Server Settings there is a “Users can only see groups they are a member of” option. If this is unchecked, users can make tasks they create visible to any group.
This is off by default because (based on the feedback I’ve received) it is fairly common for folks to give their client accounts in their Tasks Pro installation and they don’t want the client to be able to even see that other client exist in the system3.
I’ve tried hard to keep the initial user experience for Tasks Pro™ and Tasks as simple and easy as possible as I’ve been adding features for the last two and a half years, but in that time I’ve packed in quite a bit of functionality as well.