Note: this request is for multiple separate accounts (on separate plans) to be able to use the same custom domain, not multiple users/accounts on the same plan. In my case I have a custom domain I'd like my family to use (my spouse and kids, 6 total users) as well as my parents and siblings (another 6 users). Moving to a business plan is cost-prohibitive ($48/mo total for two family accounts vs. $156/mo for 12 business users), and I really don't want to have to register a different custom domain for each family plan.