We have a fraud prevention system that makes sure the people you've invited are legitimate. If they aren't then they won't count towards your referrals.
The people you invite must have earned at least 1 point from ad impressions to be verified. This means they must have completed set up including installing the extension so they can start accumulating points from ad impressions. This can take anywhere from 24hrs to 48hrs based on their web usage.
You can check whether your friends are verified by going to the "Invite a Friend" tab if they have the UK flag and two green ticks then you're good to go.