On the old site in order to be able to send someone a personal message (sometimes abbreviated “PM”), you first needed to “friend” them. Here on the new site, everyone is your friend. Simply search for them by clicking the search icon at the top right side of the page. That search tool searches, topics, posts, categories and users.
When you’ve found the user you are looking for, you can click their avatar and see their profile page.
Similarly, if you click on a user’s avatar most places, a “user card” will pop up, which also has information about them. Double click their avatar to learn even more about them.