Beneath the products added to this group page, you’ll be able to see all the items in your group. On each product’s row, there is a red cross. When you click this, this item will then be removed from your product group.
It may be that your user account doesn’t have this functionality approved so you may need to contact your system admin to get a group deleted.
Now that the item is out of your group it will be back among your other products in the add products drop-down menu.