Inviting participants to message

I can't figure out how to invite participants to a message. I've tried marking their castles and clicked the invite button but nothing shows up.

Click on their player name and it gives you the option to send them a message to add other members of your alliance to the message their is a icon in the top right corner that you can pick and choose who you want to add to it you can only add whoever is in your alliance and if you have mass mail rights

You should be able to also add people that you are in diplomacy with.

Only the member from that alliance can add them if they are not in your alliance you can't add them unless you switch alliances