\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}}*{3}{lcc}}
\toprule
 & \multicolumn{2}{c}{\textbf{group1}} \\
\cmidrule{2-3}
 & a & b \\
\midrule
\textbf{value1} &  &  \\
\hspace{12.0pt}Mean (SD) & 2.5 (1.29) & 6.5 (1.29) \\
\hspace{12.0pt}Median \char`[Min, Max] & 2.5 \char`[1, 4] & 6.5 \char`[5, 8] \\
\textbf{value2} &  &  \\
\hspace{12.0pt}a & 2 (50\%) & 1 (25\%) \\
\hspace{12.0pt}b & 1 (25\%) & 2 (50\%) \\
\hspace{12.0pt}c & 1 (25\%) & 1 (25\%) \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}