Sometimes a random member will get ‘removed’ from the party. He won’t be able to see the rest of his party members, can’t use party chat, will appear to him as though he is not in a guild, and is genuinely very annoying.
The remedy is to have the player kicked, then relog, then reinvited to the party. Nevertheless still very annoying. It has something to do with the log in servers apparently, because when it happened to me, some error prompt akin to losing internet connection has appeared, and that the login server can not be reached at this time.
Has happened to a lot of my dungeon goer buddies too.