As long as there are not too many you can send the invites to yourself (easier if you have your own domain)
e.g. firstname.lastname@example.org email@example.com firstname.lastname@example.org
This will give you the individual invitation login details that you can send to the appropriate recipients. you can then go and manually edit invitees and change the dummy email addresses to the real ones for the purposes of sending reminders.
Alternatively, create totally false email addresses. Opinio will still allocate an id and password even if the emails are returned (which they will be as they are not real). You can then use the Execute Select plugin to interrogate opinio and retrieve the id and password info that you can put into emails you create.