You can earn coins by sharing your referral link with others. Many teachers get hundreds (even thousands) of free coins every month by using their referral link.
You can get your referral link here.
Coins for a referral vary by country of the person you refer. You can check the complete country-coin list here.
You will get 10% of the coins they buy as well.
We have seen excellent work by people who refer us on Facebook. Facebook is great if you want to share with a lot of people. Posting in relevant Facebook groups can be really effective.
You can also share your referral links in blogs, forums, email footers and teaching communities as well.