Receipts are automatically sent to all meeting registrants, so invoices are only needed as a follow up for unpaid registrants. This can be done through Orders & Transactions > Orders & Invoices, but it is often more efficient to do this in Meeting Manager prior to the meeting.
Go to Meetings > Meeting Manager
Click the Registration button next to the appropriate meeting
Select Unpaid Invoice in the Status filer
Click Refresh
Click Send Invoice next to the first registrant
Click Send Invoice
Repeat these steps to send an invoice for all unpaid registrations.