'Net user /domain' queries the current domain, so you can't do what you want with that command.
Instead you can use 'dsquery user -samid ' to query other DCs/domains and pipe it to 'dsget user -memberof' to display the group memberships. dsquery command will by default query the current domain for the contacted DC. dsquery and dsget commands are available on DCs or by installing adminpak.msi
dsquery user -samid -s |dsget user -memberof -expand
dsquery user -samid |dsget user -memberof -expand