I shared the link with friends, but I am still asked to pay
To get the discount your friends have to register first (via the link). For each registration, you'll get -20% (up to a free month). You also get an email notification when the 5th person signs up.
Do I have to provide billing details?
Yes, you will see the discounted price on the Upgrade screen (make sure you've refreshed the page) and the system will ask your billing details. However, if the discount is 100% you won't be charged anything for this first month. Of course, you are free to cancel anytime before the next billing date :) (and you'll get a notification email 7 days in advance)