You donate to get LC coins. For that you have to contact Kavinsky (the owner) about your donation, so he can give you the coins, so you can buy the Members+ at the shop.
This is the donation page:
Donate Page
It costs 12$-16$.
You need 750 coins for a normal day and between 525-670 on a discount-day (those are random once a week for 24h with a random percentage between 10%-30%.
But you don't need to, unless social interactions are too much for you, you can try to earn some coins just by posting in some threads. Your post and my reaction together already gave you 10 coins.