There seems to be absolutely no user documentation to guide you through common tasks like setting up a new user with a single site, setting up a reseller or managing quotas.
We know that and we are already working on some documentation. But in general its really self explaining.
1) To create a new user, click on clients at the top, then on edit clients on the left and then on the add new client button.
2) To create a website, the same procedure just in the sites module. Click on sites at the top, then on edit domain at the left and then on add new domains.