Add new email address using custom domain for second person

I want to be able to add a second person with their own email address to a custom domain. For example (just in case it's not obvious all names and domains are fictional (otherwise I would be very rich owning these 2 domains!!)):

Myself and the second person have Apple IDs of walt@disney.com and lillian@disney.com

I have added a custom domain called mouse.com on my iCloud account

On this domain I have set myself up with an email address of mickey@mouse.com which forwards emails onto walt@disney.com

I want to add another person to the same domain, and set them up with an email address of minnie@mouse.com to forward emails to lillian@disney.com

How do I set this up?

Thanks in advance!!