My code works like this. I have n number of Lead objects. When I click a checkbox I am trying to pass the value to a javascript function. It is not working. Any help. Also How do i achieve this functionality?

<apex:page controller="registerDomainController" tabStyle="Lead">
<script>
function addUsers(userid)
{
var idsToAdd = new Array();
alert('Before adding' + userid);
idsToAdd.add(userid);
}
</script>
<apex:sectionHeader title="Add User">
<apex:form>

<apex:pageBlock>
<apex:facet name="footer">

<apex:pageBlockSection title="Add Users to the Extranet">
<apex:dataTable value="{!allLeads}" var="user" cellspacing="0" cellpadding="4" border="0" width="800px">
<apex:column>
<apex:facet name="header">First Name</apex:facet>
<apex:outputText>{!user.firstname}</apex:outputText>
</apex:column>
<apex:column>
<apex:facet name="header">Last Name</apex:facet>
<apex:outputText>{!user.lastname}</apex:outputText>
</apex:column>
<apex:column>
<apex:facet name="header">Email Address</apex:facet>
<apex:outputText>{!user.email}</apex:outputText>
</apex:column>
<apex:column>
<apex:facet name="header">Phone number</apex:facet>
<apex:outputText>{!user.phone}</apex:outputText>
</apex:column>
<apex:column>
<apex:facet name="header">Add User</apex:facet>
<apex:inputCheckbox value="{!user.id}" onclick='addUsers({!user.id})'/>
</apex:column>
</apex:dataTable>

</apex:pageBlockSection>

</apex:pageBlock>
</apex:form>
</apex:sectionHeader>
</apex:page>