Hi crmclaughlin81,

After doing some testing for your inquiry we found that currently the Project feature does not support linking multiple customer invoices under one Project. The current feature creates a Project as a sub customer and only allows transactions to be posted under that one name. If you already have existing invoices that you wish to attach under a new project you can do so by:

1. Select Invoice
2. Select Customer drop down field, select the Project name
3. Select Save & Close

This will then change the invoice assigned from one customer to the sub-customer which is the project you have created, which will then appear under the projects transactions in the Project tab. Your feedback is valuable to us and I'd encourage you to share your thoughts for this product enhancement to us by following the below steps.

1. Select the Gear icon at the top, then Feedback.
2. Enter your comments or product suggestions then select Next.
3. You'll be provided a list of suggested Help articles related to your comment. You can read through them or select Skip and send message.
4. From the drop-down, choose the appropriate category, then select Send message.

Thanks, 

-Steven