It all depends on what you want to have to haul with you to your friends’ house. If you don't want to mess with light stands and the extra setup, you could try using the 580 on camera with a light sphere and shoot that way. Here's an example of the lighting it will give without the dome on top so you get a bounce effect as well. (I used the cloud for these)
Another thing to look at is using a small soft box designed for your portable flash. Alzo digital has one that is readily available on eBay for around $59 shipped.
You can find them here. It comes with all of the hardware you need to set it up, but you will still need a light stand to hold your lights. (This is a readily available alternative to the Photoflex version that is harder to get.) Depending on the size of your group, this may work well.
If you just want an umbrella, a white shoot through can be had for cheap. If you go that route, use a local store for the stand and umbrella. Ordering online will just add shipping charges that will probably be more than sales tax on these items. Also remember that you will need an additional fitting for you speedlights to attach to the light stand and hold the umbrella so that will add to your cost.
Personally, the quickest and cheapest route is the lightsphere. It is light and easy to move around with. The light does come off a little flat, but you can work with it to get a little more shadow into the pictures. No matter what you decide to do, you should practice with the light setup at home on whatever willing victims you have until you get it right so that you don't have to make your friends sit through countless shots while you chimp at the camera trying to get the exposure right.
(One other thing I would recommend, if you are going to think about doing portraits for friends in the future or just want to make your setup quicker and have less PP after the shoot, is to get a photovision target. You can use it to quickly set your exposure and white balance on location without a lot of trial and error on your subjects. Shoot, look at the histogram, make a quick adjustment, test and you are ready to go.)