If you are a captain or vice captain of a guild you can send messages via the "Moderator Panel". When you hover over it, you should see the "Send Guild Announcement" right at the top. It costs a certain amount of gold per number of members in your guild to send.
As to "Broadcasts", honestly I don't think that even works anymore (or never really did). It had to to with the Gaia IM system that we used to have, and depending on your "broadcast" settings, you could more or less do a whole chat with your guild members (or just your crew). But once they got rid of the IM thing, it became obsolete.
Also you can now make a private guild chat space - link is also under the "moderator panel".