ServicesManager sm = Bukkit.getServicesManager();
Permission p = sm.getRegistration(Permission.class).getProvider();
List<String> result = new ArrayList<String>();
for (Player player : Bukkit.getOnlinePlayers()) {
String playerName = player.getName();
if (p.playerInGroup(worldName, playerName, groupName)) {
result.add(playerName);
}
}
return result;
}