Users on the your domain should have received a notification from Canva letting them know that their account would become part of the Side Community account. The subject line is "Your account is being added to Side Community on Canva" and the email address is no-reply@canva.com.
Within that email, there is a continue button that would allow you to change the email address. If you are unable to locate that email, please share the email addresses you'd like to use for each of the branded email addresses and we can put in a ticket with Canva support to change them on your behalf.
We apologize in advance if this process feels cumbersome, we're limited by Canva's system and settings.