When browsing to the PWDM settings in your project, under Participants-External, you will see the following note regarding Bentley CONNECTIONS Passports required for external participants

Note that all Deliverables Management users (including external participants) require Bentley CONNECTIONS Passports. If an external participant is from a non-Bentley-user organization and is not yet a Passport holder, then you will be responsible for their Passport usage, which will be automatically included into your organization’s usage report.