I guess this wouldn’t be useful enough to be integrated into the advanced search interface?
My recent use cases:
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I was looking for particular information from a Discourse staff member, but I couldn’t remember who. Filter messages from this group of users have helped.
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On my forum, I have a group in which I put business professionals. It can be useful to quickly have an overview of all their messages (as they write official information), regardless of any brand/manufacturer/seller in particular, etc without having to look at their individual message. Plus, they often work together, so the information is not always scattered between these different entities.