When creating a new post and using the @-sign with someone's name in the 'Notes' field, they will receive a notification with a link to your comment and be able to view your post. Clicking the notification brings them into the conversation.
Add a comment in the 'Notes' field.
Type your comment. Use an @mention to name specific people in your comment, and select them.
Click Send
The recipient will receive a notification. They can then view the post and make a comment on the specific post. (Make sure notifications are turned on)
