There are several ways to do this. The way I favor takes a bit of getting used to, but is fairly quick after that.
First, find whichever one of the posts you want to quote first, and press "Quote." Add whatever comments underneath that you want to, like you normally do. Do not press "Post" yet.
Next, find the next post you wish to quote, and copy whatever contents from that post you wish to quote.
Go back to your post, then type in "[quoteREMOVE] paste the post you wish to quote [/quoteREMOVE]", only without the "REMOVE"s I added.
If you want it to have the "X said:" that other posts do, type the user's name in bold followed by a semi-colon, then hit space.
Once you're done quoting all the people you want to, just hit "Post" and you're done.