In How to know if teammates are in a premade squad or not?, we learned that it's not possible to directly determine whether your teammates are in a group together, and that some people on the Battle.net forums are displeased with this non-feature.
Why would anyone care? Is it desirable (or undesirable) to be paired with a group of players? Is it considered poor etiquette to form a group of three or four people and hit quick play (so that there are one or two odd people out)?
My friends and I usually play relatively casually, so we don't (for example) worry too much about strategically combining our heroes to make them more effective. It seems to me that our presence would have a minimal effect on a game otherwise composed of individual players.