When you're logged in and viewing a thread, highlight a part of someone's text and click reply to quickly quote them in a message
To get someone's attention in a thread or post, just enter the @ symbol and type the first few letters of their username. They'll get a notification and an email that they've been tagged and that they should check the thread out.
I found out you have to tap the like text twice on my phone to get the other faces you can select. I used the new posts lightning bolt that is at the top of the screen to see the new posts.