Location attribute is important to display pageblock button in bottom alone.
<apex:pageblock title=”Members” >
<apex:pageBlockTable value=”{!memb}” var=”mem”>
<apex:column value=”{!mem.Name}”/>
<apex:column value=”{!mem.E_Mail_Id__c}”/>
<apex:column value=”{!mem.Mobile_Number__c}”/>
</apex:pageBlockTable>
<apex:pageBlockButtons location=”top”>
<apex:commandButton value=”Ok” action=”{!closePopup}”/>
</apex:pageBlockButtons>
</apex:pageblock>
<apex:pageblock title=”Members” >
<apex:pageBlockTable value=”{!memb}” var=”mem”>
<apex:column value=”{!mem.Name}”/>
<apex:column value=”{!mem.E_Mail_Id__c}”/>
<apex:column value=”{!mem.Mobile_Number__c}”/>
</apex:pageBlockTable>
<apex:pageBlockButtons location=”top”>
<apex:commandButton value=”Ok” action=”{!closePopup}”/>
</apex:pageBlockButtons>
</apex:pageblock>
Cheers!!!