You will 500 miles for the first referral and 1,000 miles for the next 9 (you can make 10 referrals per year and the bonus is good through 7/31/16).
That's a total of 9,500 topbonus miles - nearly good enough (10,000) for a free one-way economy flight within Europe. Your referral will earn 1,000 bonus miles for signing up. If you already have a topbonus account, you can create your own referral link here.
Free points are free points and the ability to earn nearly enough for a free one-way is a great deal. Flying within Europe is expensive and it's painful to burn hard earned United or BA miles (transferred from even harder earned Chase Ultimate Rewards) for an intra-Europe flights. I'd recommend signing up for an account and if you have a handful of interested friends, refer them as well.