But then a user is not a group, so only one person will receive those messages.
I think the issue here is not about who received notifications or not, not whether they are sent on the site or by email.
The issue is information overload. There should be a way to allow a group of people to receive contact email, while not being notified of every single change of trust levels. The latter initiation is important, but surely there are other ways to track it. Contact emails should be reserved to humans trying to contact humans.