@crafter If you want to use it, type @ and a few letters of the name, then wait for a list of names to appear and choose the right one. It can be a bit slow, but it doesn't work if you just type it all out rather than clicking on the name.
The person will get an alert - look at where it says 'Profile' in the top bar, with a number after it, click on it and go to 'mentions'. If you have it set in your profile it will also send you an email (subject to all the vagaries of forum emails these days).
[Cross-posted with everyone...]