That actually uses a different log in that and the store I tried them both. It's a weird circle you click on the create a new account link you get sent to a place you can't create a new account and if you go to the request access page you get sent to a log in for CIN.
I am a very new member, and I know that it took me several tries to get it right. I cannot tell you how I finally did it, but if you first direct them to cin.comptia.org, they will see at the top of the page a link to Register. Have them create their account. Once they have done that, on that same page, there is a link that says
Request Access. They have to fill all of that out, and then, as Rick Butler mentioned, it will take a few days for the Admins to approve them.
I want to mention that they will want to synchronize their passwords between all of the sites, and if they create their account for SSO (create it at login.comptia.org) they will have an easier time of things. I was in a vicious circle with my Password Vault that when I would change the password for one site, I would be told that my credentials for the other were now wrong... SSO is the way to go!
Also, out of interest, why are YOU creating accounts for them? Shouldn't THEY be doing that for themselves?
Good luck!