Today we override User Service method  - getUserCount() 
Here we create a class that extend the particular wrapper class and just provide the entries of our custom class in liferay-hook.xml so that liferay use our services instead of Original.
Step 1:-Create a liferay Plug in project and Select Plugin type Hook
   
Step 3:-Override the getUserCount()
 
Step 4:-Deploy the Hook
Deploy the hook and call getUsersCount() from your portlet :-
public void doView(RenderRequest renderRequest,
RenderResponse renderResponse) throws IOException, PortletException {
try {
UserLocalServiceUtil.getUsersCount();
  
} catch (SystemException e) {
e.printStackTrace();
}
super.doView(renderRequest, renderResponse);
}
Step 2:-Create a Hook in the Project
Right Click on Project -->New-->Liferay Hook Configuration--->Select Services
Click Next--->Click Add-->Select Service Type(UserLocalService)
For Impl Class
Click New-->Give Package Name and your Impl Class Name
Then Click Finish
This step automatically Create the entry in  liferay-hook.xml. 
<hook>
 <service>
  <service-type>com.liferay.portal.service.UserLocalService</service-type>
  <service-impl>com.ipg.MyUserLocalService</service-impl>
 </service>
</hook>
Step 3:-Override the getUserCount()
Open MyUserLocalService and override getUserCount()
package com.ipg;
import com.liferay.portal.kernel.exception.SystemException;
import com.liferay.portal.service.UserLocalServiceWrapper;
import com.liferay.portal.service.UserLocalService;
public class MyUserLocalService extends UserLocalServiceWrapper {
 public MyUserLocalService(UserLocalService userLocalService) {
  super(userLocalService);
 }
 @Override
 public int getUsersCount() throws SystemException {
  System.out.println("This text is from getUsersCount ");
  return super.getUsersCount();
 }
}
Step 4:-Deploy the Hook
Deploy the hook and call getUsersCount() from your portlet :-
public void doView(RenderRequest renderRequest,
RenderResponse renderResponse) throws IOException, PortletException {
try {
UserLocalServiceUtil.getUsersCount();
} catch (SystemException e) {
e.printStackTrace();
}
super.doView(renderRequest, renderResponse);
}
Output:-This text is from getUsersCount
Thats it every time you call getUserCount() it will call your method rather than original one.