I want to submit a session: Do all contributors need to be confirmed already?

Yes, the planned contributors should at least be informed and invited. By the Call for Participation (CfP) deadline, the contributors should also be confirmed and added to the session (please note: each contributor needs a separate account, see FAQs).

If you are organizing a session, you are responsible for making sure that all contributors enter their information in the profiles and manage it yourself, see also "FAQ Session Responsibilities". Propose only one session that can be realized in the desired constellation of people.