\pgfdeclarelayer{bg0}    % declare background layer
\pgfdeclarelayer{bg1}    % declare background layer
\pgfsetlayers{bg0,bg1,main}  % set the order of the layers (main is the standard layer)
\begin{tikzpicture}[
    simplebox/.style n args={3}{
        rectangle, rounded corners, thick,
        draw=black, fill=#1,
        minimum width=#2,
        minimum height=#3,
        inner sep=10pt, inner ysep=10pt
    },
    simpleboxtitle/.style = {
        rectangle, rounded corners=0,
        minimum width=1em,
        fill=brown!10, text=black, draw, thick,
        font=\bfseries,
        inner sep=3pt
    },
    header/.style = {%
        inner ysep = +1.0em,
        append after command = {
            \pgfextra{\let\TikZlastnode\tikzlastnode}
            node[simpleboxtitle] (header-\TikZlastnode) at (\TikZlastnode.north) {#1}
        }
    },
    coloredHeader/.style n args={2}{
        inner ysep = +1.0em,
        append after command = {
            \pgfextra{\let\TikZlastnode\tikzlastnode}
            node[simpleboxtitle,fill=#2] (header-\TikZlastnode) at (\TikZlastnode.north) {#1}
        }
    },
    user/.style = {
        inner sep=0pt
    },
    legend/.style = {
        rectangle, rounded corners=0,
        inner sep=2pt,
        draw=black
    },
    nodes = {align = center}
]

    \node[user] (misp1users) {\faicon{user} \faicon{user} \faicon{user}};
    \node[user] (misp2users) [right= 13em of misp1users] {\faicon{user} \faicon{user}};
    \node[user] (misp2users2) [right= 3em of misp2users] {\faicon{user} \faicon{user} \faicon{user}};
    \node[user,inner xsep=3pt] (syncuser) [left= 0.0em of misp2users] {\faicon{user-plus}};
    \begin{pgfonlayer}{bg1}
        \node[yshift=2pt, fit = (misp1users), simplebox={white}{1em}{2em}, header = Org. $\pmb{\alpha}$] (misp1org) {};
        \node[yshift=2pt, fit = (misp2users) (syncuser), simplebox={white}{1em}{2em}, header = Org. $\pmb{\alpha}$] (misp2org) {};
        \node[yshift=2pt, fit = (misp2users2), simplebox={gray!30}{1em}{2em}, header = Org. $\pmb{\omega}$] (misp2org2) {};
    \end{pgfonlayer}
    \begin{pgfonlayer}{bg0}
        \node[yshift=+0.5em, fit = (misp1org), simplebox={gray}{11em}{6em}, coloredHeader={MISP 1}{blue!10}] (m1) {};
        \node[yshift=+0.5em, fit = (misp2org) (misp2org2), simplebox={gray}{11em}{6em}, coloredHeader={MISP 2}{blue!10}] (m2) {};
    \end{pgfonlayer}
    \draw[-,very thick] (m1.south) -- ++(0,-15pt) -| ($(syncuser.south) + (0,-2pt)$) node [
        pos=0.25,above,yshift=-0.9em
    ] (textsync) {Sync. connection};
    \def \offsetY {3}
    \draw[->,thick] ($(m1.east) + (1pt,\offsetY pt)$) -- ($(m2.west) + (-1pt,\offsetY pt)$) node [above,midway,yshift=-\offsetY pt] {PUSH};
    \draw[<-,thick] ($(m1.east) + (1pt,-\offsetY pt)$) -- ($(m2.west) + (-1pt,-\offsetY pt)$) node [below,midway,yshift=\offsetY pt] {PULL};
\end{tikzpicture}