\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\usepackage{xcolor}
\usepackage{tikz}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{8}{lccccccc}}
\toprule
 &  & \multicolumn{4}{c}{\textbf{group1}} &  &  \\
\cmidrule{3-6}
 &  & \multicolumn{2}{c}{a} & \multicolumn{2}{c}{b} &  &  \\
 &  & \multicolumn{2}{c}{\textbf{group2}} & \multicolumn{2}{c}{\textbf{group2}} &  &  \\
\cmidrule{3-4}\cmidrule{5-6}
 & \textbf{Total} & e & f & e & f & \textbf{P-Value} & \textbf{Test} \\
\midrule
\textbf{value1} &  &  &  &  &  & 0.063 & UnequalVarianceTTest \\
\hspace{12.0pt}Mean (SD) & 4.5 (2.45) & 2 (1.41) & 3 (1.41) & 6 (1.41) & 7 (1.41) &  &  \\
\hspace{12.0pt}Median \char`[Min, Max] & 4.5 \char`[1, 8] & 2 \char`[1, 3] & 3 \char`[2, 4] & 6 \char`[5, 7] & 7 \char`[6, 8] &  &  \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}