There are very basic usergroups by default in Joomla 1.5. You will have to install something to extend the usergroups and access permissions if you want more control over what registered members can access and do etc.
Joomla has its perks, but it's wrong for your project
Joomla is smooth and easy to install, and out of the box (without changing settings in either CMS) it seems to be more powerful. But if you are trying to change anything in the way it displays and controls access, it turns out to be a brittle codebase that has practically no access control or customizable user groups.