If you have a paid plan, you can request your own users using this link: http://sharepointhoster.com/sharepoint-2013/add-users-to-your-sharepoint-2013-site
We will create your users and send them to you within 24 hours.
You can add users via SharePoint 2013 Designer: http://sharepointhoster.com/add-users-to-your-site-collection-using-sharepoint-designer-2013.html
or via the browser: http://sharepointhoster.com/add-users-to-your-sharepoint-2013-site-collection.html
If you have a FREE Sharepoint 2013 site, you cannot add your own users, but we do have an unsecure test user account (i.e. everyone has access to this account and password and could login to your sharepoint site with this account if you add it) so that you can test out the multi-user functionality of SharePoint 2013:
User Name: SPH\freetest
Password: sharept (Case Sensitive)