It is simple. Go to the affiliate page on the site https://www.coinspot.com.au/affiliate and click "Become an affiliate today".
Once you have signed up as an affiliate, there will be an affiliate link in your affiliate menu (under Account) when logged in that you can give to people that they can use to sign up with.
They will need to use this link which will take them to the registration page, and they will automatically be assigned to you as an affiliate once they register from this page.
Alternatively, if someone enters your email address at sign up, they will be added as an affiliate to your account.