Thank you @Deleted - unfortunately I'm not an administrator, so can't do the Powershell import, and can't create distribution lists - I tried using a contact group but that didn't show up when I started typing the group name.
I think the easiest workaround is going to be to create an invite code as it'll be simple to email that to all the original appointment reciepients.
Thanks for your help.
Best Response confirmed by
Jon Chambers (Occasional Contributor)