All previous messages are set as user. The key is that identification of who wrote that message is embedded within the message itself… (i.e. Aileen here. ).
A broader context of “why that approach” is embedded in the discussions here: Fun with Assistants -- getting them to really talk to each other
That said, all of the messages are not default vanilla messages; but embed the originator as metadata. This allows for programmatic extraction of who wrote what when. (i.e. for deterministic counting of all authors who contributed to that conversation)