First off, if you are looking for a critique group you need to think about what kind you are looking for? Some focus on one genre, some focus on one age range, some have no focus at all.
You also need to think about what sort of thing are you expecting to get out of it. Mainly what I look for is people of varying genres and writing styles, which might seem odd, but I think that if your book can get better by those that don't usually read your type of stuff because they look at things from a different perspective. This also works the opposite way. If they are readers of your genre they will also know what works and doesn't work in your book compared to others like it, so its a fine line to balance.
Another thing that I think is very important to a critique group. You have to like who you are working with. If you are always defensive because of something they said that you don't like, or rub you the wrong way, it will affect your critique of that persons work as well as not take their critique as seriously as you should, probably thinking, "What does she/he know?" So getting along with them is very important. You don't have to be best friends, and probably shouldn't for fear of them not being objective, but at least be cordial and nice to them.
One last thing that I think needs to be brought up, is are you looking for many opinions, or just one. I work with both myself, I like to get one person to exchange entire works with because then you get a sense of the entire thing in one fell swoop, while multiple people usually is more focused on the chapter you submitted for that week. Both have their pros and cons, so you need to decide what you are looking for again.
Oh yeah, almost forgot, do you want to do an online group, or maybe in person? Me personally I like the online ones because I have a young family so my schedule varies on an hourly basis and online allows me to fit in the critiques into my schedule instead of a set schedule once a week with live people. But whatever works for you. I haven't been in a face to face critique group since college, but would like to once my kids get a tad older.
Anything I might have forgotten or think should be mentioned, let me know in the comments. I'm off to critique some of my friends stuff now, so have a good rest of the day.