You can create a new role at any time.  This can be done in the "Role Management" tab under settings.  Simply click on the "Add new" button in the right corner and that will take you to a new page where you can enter the role name, a brief description of the role, and set up the specific permissions.

If a role exists that fits the description of this new group but you want to change it to a more appropriate name for their role, you can also edit an existing role by clicking on the pencil to the right of each role.