Typing rule for binding groups
I don't know what rule Jones intended to use, but I'd guess it's something like

$$
\dfrac{
\Gamma' = \Gamma,x_1:\tau_1,\ldots,x_n:\tau_n
\\
\Gamma' \vdash e_1 : \tau_1
\\
\cdots
\\
\Gamma' \vdash e_n : \tau_n
\\
\Gamma' \vdash e : \tau
}{
\Gamma \vdash {\sf let}\ x_1=e_1;\cdots;x_n=e_n\ {\sf in}\ e : \tau
}
$$

which handles mutual recursion in groups. The rule above does not consider patterns as in x [p] = e, but it could be amended to support those as well.

I think binding groups are usually neglected since mutual recursion can be translated into non-mutual one (e.g. exploiting Bekić's theorem). However, in a real-world language it's convenient to have mutual recursion, even if from a purely theoretical point it's redundant. Hence, having a typing rule for it and an inference algorithm is still quite useful.