Confirmations: When an invitation is sent out or offered, request a reasonably prompt reply. If the process is automated, reply as quickly as possible. Should a tentative acceptance be necessary, state when a definitive response will be provided. When an invitation has been proffered and no reasonable answer given, it is well within Netiquette guidelines to resend the request after a period of at least twenty-four hours. When initiating a second request, do so in a polite manner, without assumptions or scolding. Rather than feeling ignored, it may very well be the case that you have been the reason for the delay by virtue of a misspelling, wrongly selected email account, or aggressive spam filter. Regardless, it should never be assumed an invitee has received the request, opened it, or had the time to read it.
Reminders: The longer the time between an invitation and an event, the easier it is to have any lapses in attending. It is appropriate Netiquette to make sure that at least one reminder is sent between twenty-four and forty-eight hours of the scheduled event. If any of the attendees are traveling, make sure all are aware of this so as not to cancel or postpone without good reason. If the sender has sent at least one reminder and not had a confirmation, it is prudent either to call or send another polite message notifying the party or parties that without a reply, the meeting will need to be postponed (not canceled).