Replying to someone else's post in an individual or group stream is quite simple. At the bottom of the post you will see a Reply button:

Click this button to bring up the reply text box where you can type your message (try mentioning people to notify them of your reply!). The author of the original post, anyone who has already been mentioned in the thread, and everyone who has replied to the thread will automatically be notified, so you do not need to mention them for the purpose of generating notifications. 

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