Happens 100% if someone in group crashes trying to zone.
The not being able to see other players is not a group-exclusive bug, seen it happen on other situations too. Rarely, though.
First the leader should try to kick the bugged member, then try to disband and restart whole group, if that won't help, relog.
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Brutillus 60 Dragon Hero Bracer Shadowknight - <Awakened>
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