All you have to do is give your magic friends a special link to Magic Hat, use it in other forums as your signature, and generally spread the word however you like.
When someone who has signed up to Magic Hat after using your link gets more stars, you'll automatically get one extra star. This isn't just for when users buy stars - if they get stars when they submit a trick, you'll earn a star, if they win a competition and get stars, you'll earn a star - literally, whenever they get more stars, you get one as well!
Your magic link is http://www.magichat.co.uk/?r=your_magichat_username - make sure people type it in correctly!
You'll be sent a Private Message on the forum whenever you earn a star.