To mention someone simply type the "@" symbol and begin typing their name. Sixcycle will suggest people that match the name you are typing. Be sure to choose someone from this list, as manually typing their name out in full will not generate a mention.
You can mention everyone in the stream by using "@all". This will generate a notification for every member of the stream, so use it wisely! Again, be sure to select it out of the suggested lists of mentions. (Note: Responses to posts or replies that used "@all" will not notify everyone)
When You Don't Need to Mention
If you are replying to a post or existing reply, the author of the original post, anyone who has already been mentioned, and everyone who has replied will automatically be notified about your message, so they do not need to be mentioned.