Sub-numbering equations within array

The empheq package was made for this:

\documentclass[a4paper]{memoir}
\usepackage{empheq} % autoload amsmath
\begin{document}

\begin{subequations}
\label{eq:tot}
\begin{empheq}[left={x=}\empheqlbrace]{align}
  & a + b \\
  & c + d \label{eq:b}\\
  & e + f
\end{empheq}
\end{subequations}

Total group: \eqref{eq:tot}, just one: \eqref{eq:b}

\end{document}

The subnumcases environment from cases helps for this layout:

\documentclass{article}
\usepackage{cases}% http://ctan.org/pkg/cases

\begin{document}

\begin{subnumcases}{x=}
  a + b \\
  c + d \\
  e + f
\end{subnumcases}

\end{document}

Subequation numbering can be modified using (say)

\renewcommand{\thesubequation}{\themainequation.\alph{equation}}