documentclass[margin=5mm]{standalone}
usepackage{pgfplots}
usetikzlibrary{matrix}
usepgfplotslibrary{groupplots}
pgfplotsset{compat=newest}
begin{document}

begin{tikzpicture}
    begin{groupplot}[group style={
                      group name=myplot,
                      group size= 2 by 4},height=5cm,width=6.4cm]
        nextgroupplot[xmin=-2,xmax=3,ymin=-2,ymax=6]
                node at (axis cs:0.,0.) {$A$}; % line of interest
        nextgroupplot[xmin=-50,xmax=100,ymin=-100,ymax=50]
                node at (axis cs:0.,0.) {$B$}; % line of interest
    end{groupplot}
  end{tikzpicture}
end{document}

documentclass[margin=5mm]{standalone}
usepackage{pgfplots}
    usepgfplotslibrary{groupplots}
begin{document}
    begin{tikzpicture}
        begin{groupplot}[
            group style={
                group name=myplot,
                group size= 2 by 4,
            },
            height=5cm,
            width=6.4cm,
        ]
        nextgroupplot[xmin=-2,xmax=3,ymin=-2,ymax=6]
            % you have two choices to place stuff "relatively" in the plot
            % which yield the same result on "normal" axis
            node at (axis description cs:0.2,0.2) {$A$};
            node at (rel axis cs:0.2,0.8) {$a$};
        nextgroupplot[xmin=-50,xmax=100,ymin=-100,ymax=50,
            % but if you reverse the axis, the behavior can be adapted,
            % depending on the value of `allow reversal of rel axis cs'
            x dir=reverse,
            allow reversal of rel axis cs=false,
        ]
            node at (axis description cs:0.2,0.2) {$B$};
            node at (rel axis cs:0.2,0.8) {$b$};
        end{groupplot}
    end{tikzpicture}
end{document}