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}}