Indenting within table column

You can add the indent to all the cells in the first column (that should be left aligned) and remove the indent when you want it to:

\documentclass{article}
\usepackage{caption,booktabs,array}

\newcommand{\rowgroup}[1]{\hspace{-1em}#1}

\begin{document}

\begin{table}
\caption{Table}
\centering
\begin{tabular}{>{\quad}lcccc}
\toprule
& \multicolumn{4}{c}{Lags}\\
 & 5 & 10 & 15  & 20 \\
\midrule
\rowgroup{EUR/USD} \\
Ljung-Box-Test & 0,4936 & 0,7841 & 0,8457 & 0,8822 \\
Foo-Test & 0,4936 & 0,7841 & 0,8457 & 0,8822 \\
Foo-Box-Test & 0,4936 & 0,7841 & 0,8457 & 0,8822 \\
\midrule
\rowgroup{USD/EUR} \\
Ljung-Box-Test & 0,4936 & 0,7841 & 0,8457 & 0,8822 \\
Foo-Test & 0,4936 & 0,7841 & 0,8457 & 0,8822 \\
Foo-Box-Test & 0,4936 & 0,7841 & 0,8457 & 0,8822 \\
\bottomrule
\end{tabular}
\end{table}
\end{document}