Where do I find the "Show Addresses" option in Yosemite Mail? I have just upgraded to OS X Yosemite.

In Apple Mail I normally send email to groups. 

In my old version of Mail I would press the "Show Addresses" icon in the toolbar and select an email group, however I do not have this icon on the new version.

I've tried to customise the Toolbar, but "Show Addresses" is no longer an option.

What is the best way to resolve this?
Yosemite now has this feature built into the to field, at the end is a blue circle with a plus (+) symbol. Click this for a pop over with the Address Book.

I don't have groups so I presume this is what you need, but I can't confirm.