How to omit group separators in the decimal part when using siunitx?

The group-digits option is a choice key, which has the possible values true,false,integer or decimal. In this case, only the integer part of the number should have group digits, so use group-digits=integer.

\documentclass[a4paper,10pt]{book}

\usepackage[load=addn,separate-uncertainty=true]{siunitx}

\sisetup{%
      output-decimal-marker={,},
      load-configurations=abbreviations,
      group-separator={.},
      per-mode=fraction,
      group-digits=integer
    }

\begin{document}
    \SI{12345.66666666}{\kilo\meter\per\hour}
\end{document}

Updated due to the comment

\documentclass[a4paper,10pt]{book}

\usepackage[separate-uncertainty=true]{siunitx}

\sisetup{%
  output-decimal-marker={,},
  group-separator={.},
  group-minimum-digits=4,
  per-mode=fraction,
  group-digits=integer
}

\begin{document}
\num{62034.023458763e23} 


\num{6232.023458763e23} 

\SI{12345.66666666}{\kilo\meter\per\hour}
\end{document}