Windows domain user in Linux

[Muhammad Yousuf Khan <sirtcp@gmail.com>]

i am using winbind for samba to fetch users from windows domain. now i
want to add a windows domain user "administrator" a member of group
"root". however when i run the command it gives me an error.

#useradd -G root administrator
useradd: user 'administrator' already exists


Please Help.

Thank you,