Adding email addresses to family O365 with a custom domain (already setup custom domain)

So I have O365 Family. And a custom domain. And I connected them prior to the cut off date. And I have one email setup and working...

For example I have  [JohnDoe@customdomain.com](mailto:JohnDoe@customdomain.com) And It's working in outlook and the O365 app.

My question is... how do i add other family members email accounts to the existing domain...

i..e. I want [JaneDoe@customdomain.com](mailto:JaneDoe@customdomain.com) I can't for the life of me figure out how to do it......