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A little devil's advocate. One idea is to make invitations immediately take up a dummy spot so no one can fill it before the person has had a chance to accept. This at least helps the people only trying to bring along a single friend.
I agree it was stupid, but so what? If you're doing a guildie/friend run, premake the group or set to private. Think that one extra button of preparation will save you all the future kick clicks you'll have to endure.
You should be fine. It's contained between you two, they can see him on your friend's list, they can see your past trading activity, they can probably even bring up your conversation logs - I'm sure there's at least one dude in IT getting off to all the ERP.
I've been swapping gold and items of varying sizes and worth with a buddy of mine for years and they've never come asking questions. Don't sweat it.