\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepackage{pgf-pie}
\begin{document}
\begin{tikzpicture}
    \pie[text=pin, sum=auto]{
        15/a,
13/b,
12/c,
12/d,
11/e,
9/f,
8/g,
8/h,
7/i,
5/j
    }
    \node [above=3mm, font=\large] at (current bounding box.north) {Group participants};
\end{tikzpicture}
\end{document}