Track attendees from a Distribution List

The current flow implementation to keep track of the attendees' responses does not work with stakeholders who got invited trough a distribution list.

Start by locating the current flow implementation that tracks attendees' responses and compare its behavior for direct invitees with stakeholders invited through a distribution list. Reproduce the failure with a distribution list and trace the attendee-response handling; done means responses from those attendees are tracked correctly.

Written by the indexing model from the issue text.