Right now, invitations can only be accepted if the e-mail address of the logged-in user matches the recipient of the invitation.
It would be useful if WebMail Pro also could take into account the e-mail address aliases of the logged-in user which can be derived from the various identities which the user has set up.
If an invitation was addressed to one of the alias addresses specified in the identities, it should also be possible to accept the invitation and thus easily add it to the calendar.