I've done this a bunch. It seems to take about a six-pack's worth to get a good yeast pitch to add to a starter. I've had success doing it by just gathering the last little bit of each bottle all schwirled up and then poured all of them into one of the bottles, popped the cap on it and stuck it in the fridge and took it home after the party
Of course, the more careful you are, the better, but I have had success every time, sometimes using pretty sloppy technique. If I were to do it right, I guess I'd:
1 - Sanitize a small jar or something.
2 - pour each beer as I open it into a glass, schwirl the last bit, flame the lip of the bottle, and dump yeasty beer into said jar
3 - collect a six pack's worth of yeast and seal jar
4 - make starter
Most of the commercial beer yeasts are available through White Labs or Wyeast, but for rare, weird ones like Belgians, or just for the fun of it, harvesting yeast is a hoot.