\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\usepackage{xcolor}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{4}{llcc}}
\toprule
 &  & \multicolumn{2}{c}{\textbf{Group 2}} \\
\cmidrule{3-4}
 &  & e & f \\
\textbf{Group 1} &  & \multicolumn{2}{c}{\textbf{value1}} \\
\midrule
\multirow[t]{2}{*}{a} & \textbf{mean} & 2 & 3 \\
 & \textbf{std} & 1.41 & 1.41 \\
\multirow[t]{2}{*}{b} & \textbf{mean} & 6 & 7 \\
 & \textbf{std} & 1.41 & 1.41 \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}