When you're looking at an incoming invite, there is an "add comment" checkbox right above Accept / Reply / Decline.
That's in your incoming message that has the invite, on the attachments tab (invites are currently shown as a special type of attachment).
If you turn that on, and then hit Accept / Reply / Decline, the app will prompt you for a short note to be included in your response.
It's for things like "Can we also invite Debbie from Marketing?" or "I'll be there, but a hour later would work better".