Answer
The easiest way is to use the Appearance panel and add a stroke to either the grouped artwork or the layer that it is on; there is no need to flatten or duplicate anything then and the stroke will update with any changes to your artwork.
Use the button at the bottom of the panel to add the stroke then drag that stroke below “Contents” (otherwise it will apply the stroke to everything).
Take this handsome fellow for example:
The same artwork with a stroke added to the group:
Attribution
Source : Link , Question Author : Terry , Answer Author : Cai