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=”bottom”>
        <apex:commandButton value=”Ok” action=”{!closePopup}”/>
    </apex:pageBlockButtons>
</apex:pageblock>