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.
Packet structure
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
Possible bugs
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.