Coinbase is a centralized exchange with an extra earnings program.

By watching some short videos and answering a question about each of the videos, you earn a certain amount of the crypto you're learning about.

To spread the knowledge, some of these programs also include an "invite a friend" section. 

By following the invite link for one of these earning programs, you can earn 50$ worth of the crypt, and the owner of the link receives $10. After completing the program, you receive your own link to invite 5 more people, earning you $10 per person following the program.

Earn EOS:  Earn $50 EOS on Coinbase Earn
Earn XLM (Stellar): Earn $50 XLM on Coinbase Earn
Earn COMP (Compound): Earn $50 COMP on Coinbase Earn

Good luck and enjoy the Crypto space!