This is a sort of a glitch when you use /skin in the event-b to change your skin. When you join, sometimes the person already did /skin but in your client, the skin isn't loaded which results in the "Invisible" players. I tested with my alt and it seems like sometimes the skin is loaded properly but results in the cape missing in the skin. Sometimes, the players' skin just does not get loaded in the other players' perspective which causes these "Invisible" players. If you want to fix it, you just gotta relog and it will probably load fine. I am not certain though so I hope the glitch does get fixed.