It took me a while (more than a few seconds, in which case a regular user would be gone by then) to figure out that I'm supposed to click on the right balloon,

It took me a while (more than a few seconds, in which case a regular user would be gone by then) to figure out that I'm supposed to click on the right balloon, which is identical to the left balloon. I think you should add more contrast to your call to action. Also, as jgrahamc said, work on your message. It's not clear how much work is involved, maybe a step-by-step explanation. How are you sending those invites? What's special about your app? Do I have to pay anything?