By default all registrations through the online booking in the same session will be included in the same invoice.
As an administrator you can handle this through the super-user control panel under
In the example below you have three registrations where two are on the same invoice and the last is on an individual invoice. In order to get all three registrations on the same invoice you will have to change the last invoice number (282913) into the same as the other two (282912). All three registrations are now on the same invoice no matter what invoice button you click.