I found the solution from http://www.wandisco.com/svnforum/threads/33361-creating-a-new-svn-user-with-password?s=ea3c8ce175b09de948e8aafe7d4f8ab6&p=124540&viewfull=1#post124540.
For adding another user
#sudo htpasswd -m /etc/.svn-auth-file <username>
should be used instead of
#sudo htpasswd -cm /etc/.svn-auth-file <username>