\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}}*{2}{lc}}
\toprule
 & \textbf{group1} \\
 & b \\
 & \textbf{group2} \\
 & e \\
\textbf{group3} & \textbf{value1} \\
\midrule
c & 5 \\
d & 7 \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}