Perhaps if we change the flow it would make more sense:
Invite to a PM:
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Triggers an invite email, “Bob invited you to XYZ conversation, with a summary of the last few posts, would you like to accept” (email notification if you are not on site, green notification if you are out of the box)
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You click accept and are added into the PM as watching
I do agree that the “I can just silently add people to a PM if they want to or not” as any user is a problem, we need a different flow for people being pulled in to a discussion vs people who are there from get go.