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.
Best Answer
The reason you would care is because people who play as a group are more likely to have good synergy and teamwork, especially due to the fact that they are more likely to be using voice chat. While the game does indeed have in game voice chat, and as @DanmakuGrazer pointed out- that defaults to a private channel for groups, and very few people use in-game voice at all in my experience. They are much more likely to use something like teamspeak or curse voice as a group and coordinate strategies. I would consider is desirable to be paired with a group of players because of this, and I don't think it is poor etiquette to play as a group of people. If it were, the option probably wouldn't exist, and I personally prefer to play with my friends rather than random people. Though, if you do play as a group of 4 or 5, you may get accused of trying to pubstomp or tryharding.