How do you send mail to the entire alliance?

On the alliance screen click the envelope at the top The top field is the subject. The larger, bottom field is the body of the message.

You can send it normally if you are R2/R3 or switch to priority if you are R4/Leader. Priority is for a set duration (5 minutes up to 1 day) and costs 50,000 Alliance Gold.

With priority you get a "voting" system with buttons that are red or green. One says READY and the other - the red one - I cannot recall what it says. NOT READY maybe?