To attack someone with a hug.
To hug someone fiercely especially if the person is not aware of you until they have received the hug.
Hugging as a means of greeting before saying anything to the person.
"Hey, I know that guy over there! Let's go guerrilla hug him, you flank the left side and I'll blindside him on the right with a guerrilla hug."
"I was just minding my own business when someone came from behind and guerrilla hugged me!"