To add user, use following command
htpasswd /etc/subversion/passwd user_name
user_name with the username you wish to add in above command. It will prompt to provide password for the user.
If you are creating very first user, you need to add
–c switch in above command, which will create the file.
htpasswd -c /etc/subversion/passwd user_name
You can check existence of the file or list of configured users using following command
You might need to execute above commands as super user.