"Running around" with people means spending time with them or hanging out with them, but it's associated with doing bad things:
He ought to be ashamed of himself — running around with a girl who's young enough to be his daughter!
I never got very good grades in school. I spent all my time running around with my buddies, getting into trouble.
You can also use "run around" to talk about doing bad things, without talking about who you're "running around" with:
It seems dangerous to have all these people running carrying loaded guns.
Nancy's been running around, talking trash about me to everyone. I'm gonna kick her ass!