I like the answer you received from Usaleem. It describes some things you can do.

The fact is some people stay as free members for a short time and upgrade to premium, others will have more personal issues they need to work out before upgrading, and some may never upgrade.

13 isn't a lot. The average upgrade rate is 1 in 8, so out of 13, you should maybe expect 1 sale, and this may not happen today. Kyle has a one-time offer with bonuses and you can add more to improve the urgency.

I don't think most of us (affiliates) start out with a 1 in 8 upgrade rate. We build it, and can possibly even surpass it with time. I wouldn't spend a lot of time trying to optimize for conversions considering that you have 13 referrals.

After you have 100+, and you're not seeing conversions, then start thinking more about it because then you may have a problem.

Instead, I'd focus little by little on adding value in your messaging system to improve conversions, but mostly focus on getting more referrals.

Best wishes.

Connect with them on a personal level. If their using WA, then they have some specific goal they want to reach, all you have to do is convince them that premium will help them achieve their goal faster so they can make money faster. Pitch them their dream