If an associate creates more hours/expense/mileage entries after the invoice has been created, it can be handled in one of two ways.
If the invoice hasn't already been sent to the client, you can set the approval level back to "preliminary" and use the Refresh button. This will cause the system to look again for all items within the invoice's date range. You can then approve the invoice again and it is ready to be sent to the client.
If the invoice has already been sent to the client, you may prefer to include these entries in a future invoice. You do this by making sure that the date range of the next invoice includes these new items.
You don't need to worry that any items will appear on more than one invoice, the system knows when an item has already been added to one invoice and won't double bill them on another.
You may want to mention to the client why there are overlapping date ranges between the two invoices. If they desire, they can see for themselves that the various entries only appear once.