The Group Chat packet is used to send a message to multiple characters on a world. It is sent by the client when the client chooses to talk to it's buddylist (group), guild, party or alliance.
XX || Byte of chat type (0 = buddylist, 1 = party, 2 = guild, 3 = alliance) // unsure XX || Unsigned byte of amount of Character IDs -- For i = 0; i < amount XX XX XX XX || Integer of Character ID -- End for XX XX ... || String of chat text .
On private servers, the character ID's are not checked, as in if they are in the buddylist, party, guild or alliance. You could abuse this to DDoS a whole list of players with this packet. It's unknown if this bug is on the official servers too.