The other day, @Ntwadumela asked me to invite him to the "Islamic Content" PM by @Mr. Burgers, so I invited him. The problem is that he's on the list of invited people, but the PM doesn't show up for him. I even sent him a direct link to the thread, and that still didn't work.
Was he in the PM before? There are 2 likely scenarios here, either the pm creator changed the settings so that other people couldn't invite, or if Nutella was in the pm at some point and he left, he checked the box that makes so that people can't invite him again/pm won't show.