Please check if this covers Luton airport. If it doesn't, you pay another £2 for the bus from Luton Airport Parkway station to the actual airport. Buying the train ticket to Luton Airport "station" is an extra £1.50 and covers the bus ride.
group save means you have to travel as part of a group of three (or more). if you don't follow the terms and conditions of the ticket you will be liable to a penalty fare of £20 or twice the single fare from your originating station to the station your arrive at (whichever is greater). and "i didn't know" will not be taken as a valid excuse (unless you're a really fit bird).