Right now when you are sent a personal message, there is a "reply" button, and when a message is sent to a message group there is a "reply to list" button as well. But when you send a message to "some members of X group" (your realm, your army, your duchy, your religion, your guild, etc.), there is only the "reply" button to reply to the original sender, but not a "reply to list" or "reply to group" button to reply to everyone who the original message was sent to (assuming it was not sent with secret recipients).
This request is for adding that button to the message when a message is sent to multiple recipients who are members of the same group, but who do not represent the whole group.
Benefit: player convenience; to reply to the conversation group a player has to open the message interface, select "send to some members of X", then select (and probably double-check in another tab) the characters that were tagged in the original message. With this feature, the player would simply click the "reply to group" button and the recipients list would be appropriately populated.
Possible exploits: none that I can identify.