If they register the domain from you and you operate the DNS server, then you have the information to make changes for the user. I think that is what google doe

If they register the domain from you and you operate the DNS server, then you have the information to make changes for the user. I think that is what google does. If the users own their own domain via a third party, you can not modify their DNS record unless they gives you their account. Or user can transfer domain to you.