I'm trying to do a similar thing. I have created the user in the security tab and set the access on the page to be viewed only by logged in users.
The problem I'm getting is when I try and log in as the user it keeps telling me I can't access the cms and doesn't log me in.
Is there something in the admin I need to set to say this user shouldn't access the admin, but can log in?
I'm having a similar problem (using the latest Silverstripe). I've created a menu, made some of the pages visible only for a special group in the security panel, however something goes very wrong. This is what happens:
When you log in, not as an admin, but as a user in a special group, you get redirected to a page I've set using this tutorial from ssbits: http://ssbits.com/custom-login-form-with-group-based-redirection/ - this part works fine.
Now logged in as a user in this special group I can view the secured pages (this is possible since I'm getting redirected to one of them), but if I refresh the browser or just clicks on any other menu link, the user gets logged out and all the secured menu links gets hidden again.