One of the popular aliases in other IRC clients is ‘/amsg’. This commands sends its argument as a message to all the currently active channels.
An implementation of this command for ERC is:
(defun erc-cmd-AMSG (&rest args)
"Send a message to all the channels of the current server."
(when args
(let ((message (substring (mapconcat #'identity args " ") 1))
(channel-list (and erc-server-process
(erc-channel-list erc-server-process))))
(when channel-list
(dolist (channel channel-list)
(with-current-buffer channel
(erc-send-message message)))))))Add this to your Emacs or ERC startup file, and then you should be able to type
/amsg Hi, everyone! Good morning :-)
to send a message to all the channels you have just joined.