Is it possible to message everyone on my Xbox One friends list with one message?

yes you can. when it says add recipient, just select all of your friends. Please also keep in mind though, they will see who else you send it to, so if you have friends that don't like each other, I would be careful.

You can pick multiple recipients when composing a message.

Thanks for the help there. But what happens when I am too late pressing the Home button when a notification shows up?

I think there needs to be a way for sending/accepting invitations or messages needs to be streamlined. When I am playing a FPS like Titanfall, I can't just leave my character standing still in the middle of the battlefield while I back out to the home screen.
I think there needs to be a Kinect ability where you do things like this. Say you are playing in an intense Attrition game and you see your friend got on and you want to invite him. You should be able to say "Xbox invite Person 1." Person 1 being a friend
that you have set as Person 1. Almost like speed dial on a phone.

All notifications go to the notification center (top left corner of Home). If you miss acting on a toast (party invite, friend coming online, etc.) because you're in a match you can wait until it's over, go Home, go to notifications and you can join there,
open their profile etc.

You can actually do what you're talking about using voice though it's a couple more steps and you'll still get some UI over your game...

Here's an example:

You get a party invitation from your friend. When you see the toast say "Xbox, show notification" and then "Launch." This will launch the party app and accept the invitation so all you need to do then is say "Xbox, unsnap." At that point you should be in
a party with your friend.

Another example:

You see your friend come online (starting in April) but you don't have time to act on the toast. You can say "Xbox, invite" which will snap the party app, "Invite friends," (you will get UI over your game unfortunately), find the person in the list and say
"Person 1," and then invite them to a game/chat party. Then you can unsnap the party app again with "Xbox, unsnap" and you're GTG.

Give these a try and let me know how it goes. I really like the suggestion about being able to invite someone directly when you get a toast, I'll take that to the team. Thanks for the feedback!

There are a few different ways you can do what you're taking about. You can snap the activity feed to see what your friends are doing. If you press the system menu button when you have a friend selected in the Friends app you can message them directly. When
sign in notifications show up you can hold the Home button and invite them to your party/game from there. We're aware that bringing up someone's profile over a game is sometimes slow; we're actively investigating it.

Thanks for the feedback regarding 360 integration (I completely agree). I'll pass it along.