Code:
#include "stdafx.h"
#include "setup.h"

       
  void OnGuildJoin(Player * pPlayer, Guild * pGuild)
 
  {

if(pPlayer->GetItemInterface()->GetItemCount(5976, false)>= 1)

  {    
  
             pPlayer->BroadcastMessage("You already have a tabard.");
 
  
         } else {
 
Item *  pItem = objmgr.CreateItem(5976, pPlayer);

      pPlayer->GetItemInterface()->AddItemToFreeSlot(pItem);
      pPlayer->BroadcastMessage("You're an official guild member now!");
 }
   }
 
       
void SetupOnGuildJoin(ScriptMgr * mgr)
{
    mgr->register_hook(SERVER_HOOK_EVENT_ON_GUILD_JOIN, OnGuildJoin);

}
Have fun.