Populate record list based on the picklist value chosen?

<apex:page controller="harishrao" >
<apex:form>
<apex:selectList id="countries" value="{!Group}"
        size="1" required="true">
 <apex:selectOptions value="{!countries}"/>
</apex:selectList>
<apex:commoundbutton value="show" action = "show" />
</apex:form>
​</apex:page>

public class harishrao
{
public list<user> usa group 
public string group {get;set;}
public List<SelectOption> getCountries()
{
  List<SelectOption> options = new List<SelectOption>();
   Schema.DescribeFieldResult fieldResult =
 User.Group__c.getDescribe();
   List<Schema.PicklistEntry> ple = fieldResult.getPicklistValues();
   for( Schema.PicklistEntry f : ple)
   {
      options.add(new SelectOption(f.getLabel(), f.getValue()));
   }       
   return options;
}
public pagereference show (){
usa = [select id,name form user where Group__c = :group   ]
return null ;
}
}