Liferay was built from the ground up to be used by application service providers. Because of this it supports having multiple portal instances in a single installation to obtain a complate isolation of the users, organizations, locations, communities and any other data created through Liferay's portlets is achieved.
The following is a sample list of portals running off of one portal instance hitting one database and shows the capabilities of Liferay: http://demo.liferay.net, http://my.ccuc.net, http://my.3sixteen.com, http://www.gatewayfriends.org, http://www.jasonandiris.com. Users in each of these portals have no information about the other portals. They are separated by domain and each portal exists in its own space based on the company's id.
Starting with Liferay 4.3 it is possible to create new portal instances directly from the web UI with no need to restart the application server. Also this method works with any application server. The creation and administration of portal instances is done through the admin portlet. Here are the steps necessary to create a new instance:
Preparation: make the necessary DNS configuration to assign a new domain to the server where Liferay is installed. Make sure all the necessary changes to external software (web servers, load balancers, firewalls, ...) have also been done. For these instructions we'll assume that the domain alpha.com has been set up.
Access the portal as an administrator and go to the Admin portlet (for example, add it to the user's personal community if it's not there already)
Click the Instances tab and then click the Add button.
Fill the form fields:
Web ID: The identifier that will be assigned to this instance. It is recommended that the domain name is used. For example: alpha.com
Virtual Host: The domain that will be used to access this portal instance. For example: www.alpha.com
Mail Domain: The domain that will be used to assign email addresses to the users of this instance. For example: alpha.com
Click save
Access the newly created instance in a different browser window. In our example you can do it through the URL http://www.alpha.com (or http://www.alpha.com:8080 if you are accessing tomcat directly and is running in its default port)