How to link items table field options to other custome fields

i have created one custome field in sales invoice, i have to link the items table information to custome text field please tell me how to do this.
example:- In sales invoice we can select item, item is grouped.
if user selects the item, attached custome filed should be display below the item table. i have created the attach field,but how to link this to items table information dont know. guid me team.