When a message has an invite attachment, it will be show in the attachments pane (swipe left from the right edge to see it, as with all other attachments).
You should see a preview: who, what, when, participants, etc.
Tap the preview to open AquaMail's own invite processing window. That has buttons / options to 1) send a reply to the organizer 2) save in one of the device's calendar-enabled accounts.
