I'm not sure what you're trying to join... You've posted so that means you've joined the site - MedHelp; that's all you need to do. As Paxiled said: you can now participate without joining, but if you want to join something let me know what it is and I'll try to help you.
It doesn't matter -- you can participate without joining. Joining gets you more emails, but it doesn't affect how much you can participate.