No — each conversation runs strictly between the buyer company and that one supplier company; nothing you write reaches another supplier.
A demand carries a separate conversation per supplier. On the buyer side the Messages tab lists them under Conversations on the left; every row is a single supplier company, and what is said in that row stays with that company. Suppliers get no such list: a supplier opens only their own conversation, from the Messages button on the offer screen, and never sees that other suppliers have conversations at all.
That boundary is price secrecy carried through. MarbleMap runs on sealed bids — a supplier sees neither rival offers, nor their prices, nor even how many offers arrived. Messaging sits inside the same wall.
Only people from your own company can be brought into a conversation; nobody from outside can be added, and there is no way to message anyone outside this demand. To check who is present, tap the participant avatars in the conversation header to open Participants.
Also asked as: Who can read my messages? · Is the demand chat private? · Can rival suppliers see what I write? · Does my message to the buyer go to other companies?