How to make all players send a message?

Loop through all players and make them perform a command which sends a message? Or what do you mean?

 

Just:

Code (Java):

Bukkit.getOnlinePlayers().stream().forEach(player -> player.chat("The message you want to send"));

This is using Java 8's streams. If you want it Java 7 compat, just loop through all online players using an enhanced for-loop.