If there's room, I think a separate "posterid" column would be great.I'd probably like that better than adding clutter to the email column. At least it would be a different number in each row.
But from a UI design standpoint Mark should likely be moving towards making it unnecessary to see and use arcana such as the posterid number, useful though it is right now.
Maybe a column of tool icons, a magnifying glass for "All Posts by This Member" and an envelope for "Send Message".
Yes, I know that Send Message is in the Actions menu, but I'm probably not the only one that finds that non-intuitive when you want to send a message to a single individual. Yes, I know that "Send Message" is already a button on the subscription page, but so is "Activity History" and that doesn't seem to be enough for some.