But wait; why are you looking to buy anything? Just for a 5 person portrait in a family room?!?
Studio lights are overkill, IMO. The reality is, if you have neutral colored walls, just bounce your flash off that. Your light will be much bigger and softer than any softbox or umbrella. If your walls aren't neutral, just get a 36" or so white shoot-through umbrella. Set your 580 to M, also manual zoom set to the widest setting, I'd even flip down the little diffuser. The point is, your 580 more than powerful enough and you can make it's light just as beautiful as a studio strobe.
When I'm shooting group portaits at a wedding, I'm working with a single off-camera 580 shot through only a 22" umbrella for portability. And those portraits are shot in dark churches or banquet halls. Here's a portrait from a couple of weeks ago of 19 people done with only one 580 and my little 22" umbrella:
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Certainly, more is always better. I could have made better light with one of my Calumet 400's (I personally prefer them to AB) and a 50" softbox, but that's just not practical when shooting a wedding on location.
If you want to get into bigger and more complex lighting because you enjoy the hobby, that's one thing. But if your only objective is to take a nice photo of five people in your family room, first think about bouncing, then think about your 580 through a rather inexpensive umbrella.