Hey Chris,
Your best bet is to create linked fields on the Invoice table that bring over fields from the InvoiceLine table, then base the grid around the Invoice table and share all of those fields. You can create a linked field for every field on InvoiceLine that you need. Now, based on what your'e describing I'm assumign you want a grid of all the invoiceline items you have assigned with the invoice information, correct? In that case, you likely want to do the opposite, and add the AssignedTo, Customer and TxnDate of the invoice as a linked field on the InvoiceLine.
Let me know if you need further clarification on this one. I can send over some screenshots of the process if that will help.
Naeem