There are several user access plugins available, including one called Members.
The settings panel for the Members plugin is in Settings >> Members Components. From there you can enable what you want. To edit roles you have to enable the Edit Roles component, then go to Users >> Roles. Click on the role you want to edit. You will see a list of all available capabilities with the ones that are enabled checked and in bold. There are six text boxes at the bottom of the page allowing you to add six capabilities at a time. Just type in the names there and they will be added to that role and to the list of capabilities. To add them to another role, just select them in the list.