\begin{tikzpicture}
\begin{umlsystem}[x=4, fill=red!10]{The system}
\umlusecase{use case1}
\umlusecase[y=-2]{use case2}
\umlusecase[y=-4]{use case3}
\umlusecase[x=4, y=-2, width=1.5cm]{use case4 on 2 lines}
\umlusecase[x=6, fill=green!20]{use case5}
\umlusecase[x=6, y=-4]{use case6}
\end{umlsystem}

\umlactor{user}
\umlactor[y=-3]{subuser}
\umlactor[x=14, y=-1.5]{admin}
\end{tikzpicture}