Well let’s see …
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sending a PM requires trust level 1
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sending a group PM requires trust level 2
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the right for someone to voluntarily opt remove themselves from a PM they don’t want to be on, at any time → incontrovertible
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at trust level 1 you can mute users at will, which prevents them from sending you PMs or adding you to PMs
Also, I’m unclear why you would add someone to a PM, and then urgently need to remove them? And if you did… why not flag like any other time things get urgent?
In other forum software there is an option to indicate when you start a PM if it is an “open” PM that anyone can be added to, or if the list of people in the PM is static after creation and cannot be changed.