pcfranchina,
I clicked the link you gave to the listing for the lens over at B&H. Hadn't done that when I wrote my first response.
I generally don't pay very close attention to the review when there is only one, unless it's thorough enough to give me some confidence that the reviewer knows what he's talking about. But this lens has two reviews, both very enthusiastic. Bottom line: It seems like it's at least as good as the Pentax 18-55 kit lens, which is already not a bad lens.
I note also that the price ($150) is quite good. Remember that every purchase you make - whether it's a lens, or a body, or whatever - involves compromise. And compromising a bit on quality in order to save money certainly makes sense. The
17-70 macro that I have costs $400. That's more than twice as much as the lens you're considering, and I doubt that the better lens is more than twice as good. It's the nature of these things that, at some point, you start to pay a significant amount of money for small amounts of extra quality.
So my impression is, the lens you're considering looks like it might be a decent purchase. Appears to be an improvement over the kit lens, in terms of range especially at the telephoto end, and also in terms of speed (the kit lens only goes to f/3.5, if I recall correctly), and it might be marginally better in terms of image quality, if the reviewers at B&H are right. Check out B&H's return policy carefully. But if you don't want to spend more than $150-200, this looks like it would be okay for the purpose you described (shooting parties).
By the way, although I don't know the Pentax 28-80 that you mentioned, I would not consider it. It's longer than the kit, but you lose the wide angle, so in terms of range, it's got no real advantage: it's different, but not better. And it's slower than the Sigma you're considering.
Good luck.
Will