On Earn.com you can earn bitcoin by replying to emails and completing tasks. You can keep the money, or donate it to charity.
Only by setting up your account, you earn $1 (0.0001564 BTC, $1 = 6,395.14). In addition, for each referral, you earn $1. 

So, you may think: let use the referral link with 100 emails to obtain $100.

Why mass referral is not possible:

• You can’t cash out anything if an account is not verified.
• You are not receiving any referral if the invited user is not verified.
• To be verified you need:
  – A work or school email;
  – Connect LinkedIn account with over 50 connections!
  – Connect Facebook account;
  – Connect Github account

Conclusion: do not loose your time trying this strategy!