If you type their name in the search box, it should give you a 'lozenge" thing that you can select from: a particular person. This is to search by, not to sort by.
It's easy to sort by a column if you're using the "classic' view with the previews at the top of the screen. With the "three pane" view with the previews at left, I'm not aware of any easy way to sort.
When you type into the search field, Mail identifies all categories the text belongs, People, Subject, Mailbox, etc.
It displays those categories below the search field. If you pick one, it limits the search to exactly that combination. If you don't select any of them, it searches the entire message.
You sort the messages by using the sort field at the top of the message list.
If you are not getting all the categories to choose from, and the search phrase should be generating category matches, then you need to re-index Spotlight.
OK - I get it now. The "People" category threw me, because I was trying to search and sort for all the emails from a particular company. I think they should rename the "People" category the "From" category, 'cause emails aren't always from individuals, but companies, newsletters, advertisers, etc.
I say "search and sort," because I not only wanted to find the emails from the company in question, but sort by date, and that part is pretty simple just by clicking on the date sent column.
Thanks so much Barney and William!
A year later, ran into same problem when going from snow leopard to mavericks. The search brings up way more items than I am looking for. Turns out the search by "from" and "to" and "subject" are still there, they are just hidden. (don't know what apple genius decided to hide useful buttons--how many safari users never figure out where the download "ghost box" is?). Here is how it works. You put your search term (george) in the mail search box. As the search is going on, a menu will appear underneath, saying "message contains george." BUT if you then click on those words, a new option will appear in the search box, with a drop down menu that has the old "from" "to" subject" etc. options. Guess the old way was just too easy.