Typing @ in a Comments block's write box shows a short list of people to mention, and picking one, or simply typing their full email, notifies them once your comment posts, the same in app notification (and real push, if they've opted in) your app already delivers everywhere else.
Who can be mentioned
Only people who have already posted in that same comment topic show up in the suggestion list, or can be mentioned by typing their email directly. This is intentional, showing every registered visitor's email to anyone typing "@" would let a stranger discover who else uses your app, mentioning stays scoped to people already part of that conversation.
Tip: A mention that doesn't match anyone (a typo, or someone who's never posted in that topic) is simply left as plain text in the comment, nothing breaks, no notification goes out.