Try this.
Index: java/org/l2jmobius/gameserver/model/Party.java
===================================================================
--- java/org/l2jmobius/gameserver/model/Party.java (revision 8661)
+++ java/org/l2jmobius/gameserver/model/Party.java (working copy)
@@ -360,6 +360,10 @@
msg.addString(player.getName());
broadcastToPartyMembers(msg);
broadcastToPartyMembers(new PartySmallWindowAdd(player, this));
+ for (PlayerInstance member : _members)
+ {
+ player.sendPacket(new PartySmallWindowAdd(member, this));
+ }
// add player to party, adjust party level
_members.add(player);