This glitch occasionally happens. When you see someone who is invisible be sure to /rejoin. That person will hopefully be visible to all once you have rejoined. If they are not, then I suggest you tell someone like a Moderator. This glitch usually happens when someone walks into a room, or if they are playing a game. I saw this happen at the library once and I just rejoined and it was okay. I hope this helped.

~Angi