User has several projects for which they want to email the invoices to multiple contacts. However, Core only has the option to set up one Billing Contact per project in Project Details > General > Billing Options. How to send the invoices to multiple contacts?
When you set up the Billing Contact for a project, you have the option to select one of the client contacts that you have created in the Client Details > Contact screen. If you also select the Send Invoices to Contact option, Core sends the invoices of the project to the email address of this contact instead of the default client email address when you email the invoices. Because you have the option to choose one Billing Contact only, you cannot set up Core to directly email the invoices to multiple contacts. However, as a workaround, you can always add multiple email addresses for the client contact that you have chosen as the Billing Contact. You can do that by editing the client contact and using the Add Contact Method option in Client Details > Contact > Contact Details. Check the Core Help for details.
When emailing the invoices to the billing contact, Core emails them to all the email addresses defined for it. You might also want to check out these articles: