Doesn't look like there is a way to select them all and add them all via the editor GUI. So if you want some rapid way of creating groups, you may want to manage it at runtime in script.
For example inside a parent node, say you want all it's children to be grouped you could write something simple like this:
for child in get_children():
You will find group methods in the SceneTree: http://docs.godotengine.org/en/stable/classes/class_scenetree.html