Are you using different membership provider for the signup control? If you are, you should not be able to log in the administration, because the newly created user is not valid for the default (cms) membership provider.
My second guess is that you are using the Community edition. If this is so, you won't be able to manage users as this functionality is not included in the free version. You can't manage roles or users, and that is why the Users and Permissions sections in the administration are missing.
The "Forums" module is a strong example why permissions and users are needed. We are considering some changes that would allow proper work of the forums, including registered forum users, as well as some general improvements in permissions. However, it is still not clear which changes will make it for the January release.
the Telerik team