Basically, if you refer a friend, colleague or anyone they can use your discount code when starting a subscription. You send them the code, they use it when selecting a subscription. Win win situation.
This question has two answers, good news and good news. You can add people during the onboarding questions, by adding their email address. Another option, is to go to account settings and go to User management.
Changing your password can be done in the account page. Go to your profile by clicking the account icon in the lower left corner. Now browse to security and you'll be able to change your password.