add a custom domain to one user in my family plan

With a Microsoft 365 Family subscription, a custom domain can only be connected to Outlook.com if the domain is managed by GoDaddy, and the feature is for creating a personalized email address on Outlook.com. It is not configured per‑user inside the Microsoft 365 admin center like a business tenant; instead, the supported scenario is:

• Use a GoDaddy‑managed domain.
• Connect that domain to Outlook.com for a personalized email address (for example, ******@customdomain.com) as part of the Family/Personal subscription.

At this time, only GoDaddy‑managed domains are supported for this consumer feature. Other registrars are not supported for connecting a custom domain to a Microsoft 365 Family or Personal subscription.

So, to configure this for one family member:

1. Ensure the domain is registered and managed at GoDaddy.
2. Use the personalized email feature for Microsoft 365 Family/Personal to connect that GoDaddy domain to Outlook.com and assign the desired address (for example, to that family member’s Outlook.com mailbox).
3. Other family members can continue using their existing addresses; only the one user needs to claim an address on that domain.

If the domain is not with GoDaddy, it cannot be connected to a Microsoft 365 Family plan; a Microsoft 365 business subscription with full domain/DNS management would be required instead.

References:

• Domains FAQ