Cygwin does not have
adduser. Instead, you should first create a user in Windows and run some commands in Cygwin to read the user list.
Create the user in Windows (Usually Start|Control Panel|Manage User Accounts|Add...)
mkpasswd -l > /etc/passwd and
mkgroup -l > /etc/group to get all of the local users and group synced up. Otherwise, replace the
-l with a
-d to get all users and groups from the domain.