If that is the case, In not in favor. Players have private messages with another player or groups of players that are no concern to the guild founders or leaders. I as a guild founder / leader of guilds at different worlds, would not want that ability.
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If the game provides to mark a message at the time of creation as an official guild message, then to allow all guild leaders to have admin rights to that message is ok.
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If reffering to the Guild Chat, available only at the PC version of the game, then we can explore the idea. What is the meaning of an "administrator" of the chat? Need more details.