$\underline{G}$ is the homotopy loop space of $BG$.

More precisely, the two terminal maps $G\rightarrow pt$ and $G\rightarrow pt$
yield a weak equivalence $\underline{G} \rightarrow  pt\times_{BG} pt$,
where the right side denotes the homotopy pullback of the diagram $pt\rightarrow BG\leftarrow pt$
and $pt$ denotes the representable stack of a smooth manifold given by a single point.