You must ensure that the contacts you want to add to your group are stored on the phone memory and NOT sim card. If they are not on the phone you must copy them to the phone memory.
Go to contacts, left menu, choose Import/export then choose the " import from sim card " Add the contacts you want to copy to the phone memory. Once this is done go to the group and click left menu and add contacts and you will now be able to choose the ones you want in your group.
Hope this will help u guys who have problems with this.