I have explained this in the FAQ
I am copy – pasting that answer here but be sure to check out the FAQ to learn about other stuff.
Q. How do I respond to someone in a thread?
Simply use the @ symbol followed by their username. This automatically provides a link to their last quip on the thread. But remember, for the @ linking to work, the person must have written on the thread already. When you type in ”@” (without the quotations) in a quip, you should see a list of people you can respond to. The list is composed of every username that has written on the thread so far. You can either choose from the list, or type in the username on your own.