Working on games

I went to a game meeting a week back and made some new friends and got a chance to look at their stuff. Some of them said they wanted my opinion on things. Holy Crap! That's awesome. My biggest issue? I have to be careful about what I say. This is their project, not mine, so how do I input my opinion without hurting theirs? I don't want to come across like a glory hog or as a...I don't know...someone who takes control of things and takes them from other people. I hope I get a chance to learn that.