On the bottom right of every post there are three "buttons"
The first one says "quote" on it and you can select that button to quote just one person. It will take you automatically to your reply with the selected quote.
If you select the button immediately to the right of that one (it's got a quote symbol " and a plus sign +) that will let you quote multiple posts in a thread.
The last one is a "quick reply/quote" button and will quote the selected post in the special box at the bottom of the page. I rarely use that one, but I use the other two often.
Hope that made sense!