Let's suppose you have a number of items that can be sold as retail items or as tester items. The tester items are the same physical product but the packers will need to adhere a "tester" sticker or prepare the item in some way, so they need to distinguish the tester version from the regular version.
So we have two versions of the same item, each with their own SKU, but the barcode is the same and it's important that the user be able to distinguish between these SKUs during scanning.
Before we look at the setup let's consider two cases that will not actually require a shared barcode. In other words, cases where shared barcodes would not be used:
Case 1
You have the same item sold under two different SKUs, using the same barcode, and you do not need to distinguish between them, then you likely just need to Alias the items, which is to bring all the SKUs together into the same product record so the barcode can be shared. That is covered in this article on Aliases.
Note: If the SKU is different but the barcode is the same, it is possible to Alias the tester and regular versions of the SKU together into the same item allowing them to share a barcode but there would be no indication to the user that a tester product was ordered, and GroovePacker could not distinguish between the items. Two say it another way, If they are aliased they are ultimately the "same product" so it's no longer possible to show the user a special message etc. when the "tester" version is ordered.
Case 2
If a separate SKU and barcode are to be used for the tester and the tester SKU appears as the line item in the order, then there is no need to take the steps below. You can simply set the tester up as a separate product.
Set up a Shared barcode
So going forward we'll assume that the same barcode must be used for two (or more) separate SKUs and that the SKUs must remain independent products.
Disclaimer: Since it's the barcode that the software uses to distinguish one item from another, once we allow the same barcode to be added to multiple items, the software can no longer distinguish between them, ie it can no longer let the user know if an accidental substitution has occurred. Shared barcodes should only be used in cases where you're willing to give up the software's ability to catch errors for the items which are sharing a barcode.
As it turns out there is a novel way to set this up.
Keep in mind this solution can only work if the SKU of the tester item that appears in the order is not the same as the SKU of the regular item. It is this difference that will allow GroovePacker to differentiate between the two items.
Setup:
Now that we've verified a shared barcode is needed and the consequences are understood, the setup is super easy. This may actually be a case of "easier done than said" :)
During a product import, if two SKUs share the same barcode GroovePacker will automatically prevent the barcode from being added. So, if you have two SKUs, one with the barcode, and one where it's missing, all you have to do is manually try to add it to the second item. If the "tester" SKU has not been created, use the "create product" option in the products section to add the SKU.
So now you should have two SKUs, one with the barcode and one without. When you try to add the barcode to the one without you'll see a popup that asks if the SKUs should be aliased. Choose "no, they are different items". Next, it will ask if a Shared barcode is required. Here, choose the option to create a shared barcode.
1) Manually add the same barcode to a second SKU
2) In the popup choose, the first option, to Not Alias:
3) Next, choose the first option again to Create the Shared Barcode 
That's it! You now have two separate items sharing the same barcode.