This IRC operator only command sends a memo to all registered NickServ accounts. This command is generally considered quite intrusive, and this command will always fail on accounts that already have exactly 20 memos. As such, this command is not the most reliable method of ensuring that all registered users get specific information. Consider using Global instead. This command accepts 1 argument, the text of the memo to be sent. To send a memo saying "Hello, World!" to all registered nickserv accounts, you can use this command:
/msg MemoServ sendall Hello, World!