Installing the Security Connector into Salesforce creates a Sitecore section in the contact or lead record (depending on the package installed). When a new a contact or lead is added, a Sitecore user name is automatically created in the SitecoreUsername__c field in the Sitecore section.
By default the value inserted into the SitecoreUsername__c field is a combination of the first name and last name. If this combination is not unique, a suffix is automatically added. You can change the behaviour to use another Salesforce field (like email address) by going to Salesforce.com > Setup > Develop > Custom Settings > Manage S4S Settings. Once in the Edit Settings screen, create or edit the "DefaultSitecoreUsernameField" custom setting name to another Salesforce field name like "Email".