Office 365 (Known Issues) - PantherLists Not Recognized for Meeting Invitations
PantherLists are email reflectors, not Microsoft Groups.
Unfortunately, as an email reflector, Outlook is unable to expand the list to show each list member or their availability. Meeting organizers can add a PantherList to a calendar invitation and they will receive the meeting invite, but the organizer will not know who is and is not planning to attend.
As a workaround, meeting organizers can enter each attendee in the meeting invite or create a contact group containing all attendees and invite the entire group.
Instructions for creating contact groups are available on Microsoft's People overview page under the "Create a Group" heading.