It doesn't matter -- you can participate without joining. Joining gets you more emails, but it doesn't affect how much you can participate.
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.