user field in visualforce page

<apex:page standardController="Account" extensions="AccountListViewController" recordSetVar="accounts" >
<apex:form >
 <apex:pageBlock >
    <apex:pageBlockSection title="Select User">
        <apex:inputField value="{!userId.ownerId}"/>
    </apex:pageBlockSection>
<apex:commandButton value="Send Email" action="{!SendEmail}"/>
 </apex:pageBlock>
</apex:form>
</apex:page>
->My controller
public with sharing class AccountListViewController {
    Public Account userId{set;get;}
    List<Account> selectedList;
    public AccountListViewController(ApexPages.StandardSetController controller) {
       userId=new Account();
	   selectedList=(List<Account>)controller.getSelected();
		system.debug('selectedList--------------------'+selectedList);
		if(SelectedList.size()>0)
		{
			userId=SelectedList[0];	
		}
    }
    public void SendEmail(){
    if(userID.ownerID!=NULL)
		User u=[select email from user where id=:userId.ownerId];
    System.debug('========================='+u);
    //Messaging.SingleEmailMessage msg=new Messaging.SingleEmailMessage();
    //msg.setSubject('Selected Records');
    //msg.setToAddresses();
    //msg.setToAddresses(new String[] {u.email});
    //msg.setPlainTextBody(selectedList);
    }
}