It is not so common to see airlines giving out miles for free just to sign up to their frequent flyer program and their newsletter, but Airberlin is doing just that right now. You will get 1000 miles just for signing up with them and subscribing to their newsletter, and you can get an additional 1000 miles for each person you refer and sign up as well. (you get only 500 miles for your first referral, but 1000 miles for each extra referral thereafter.)
If you want to refer one or more of your friends, you can follow this link to get either your referral link or to email them. If you are not an AirBerlin topbonus member but would like to join, you can use my referral link and we will both get 1000 miles each, and you can then get your own link to start referring your friends to get more miles.
Apparently the promotion is limited to 10 referrals, but that still means if you sign up yourself and refer 10 people, you will get up to 10.500 miles, which is enough for a free ticket within Europe (see the award chart here )
Overall a very good deal, and the best thing is your referrals don’t even have to fly airberlin for you to get the bonus, all they need to do is sign up and use your link, and you are good to do. Even if you don’t want to fly yourself, Airberlin has a online shop for their miles we well, and you can get something from there instead of tickets if you want. Airberlin topbonus online shop