Participants unable to join large meeting

On two separate meetings multiple participants have been unable to join because they got this error message:

AADSTS50020: User account '(email)@gmail.com' from identity provider 'live.com' does not exist in tenant '(company name)' and cannot access the application 'https://urj.zoom.us'(Zoom Web Portal) in that tenant. The account needs to be added as an external user in the tenant first. Sign out and sign in again with a different Azure Active Directory user account.

how can we resolve this?

thank you!