A Lie -groupoid is supposed to be an ∞-groupoid internal to a suitable category of generalized smooth spaces; see internal ∞-groupoid.
Following the general idea of space and quantity and generalized smooth spaces one may identify smooth -groupoids with smooth infinity-stacks.
But notice that the traditional notion called Lie groupoid is more restritive than a 1-coskeletal smooth ∞-stack.
The details usually depend on which aspect of higher Lie theory one wants to capture.
A systematic setup is proposed at ∞-Lie theory.