I'm wondering why you decided to avoid regestration for you users? Yiu know, we all get used to to do this and I have some starnge feelings abot that like it's not safe or something..maybe I' paranoic a bit 
I think that it's even no need at such services ..Through the website I can see that they have strong sponsors and a couple of my acquintances are using it. So, I think you don't need to worry)