In Tracker comments, you can ping somebody by simply doing @username. The problem is you must be aware about this. Something like autocompleter poping up after typing @ would be nice, similar how it works here on Phabricator.
Implement the feature.
Student is expected to send a patch for wikimedia-cz/tracker repository, hosted at Wikimedia Gerrit. When claiming task on GCI website, student should claim a respective Phabricator task as well.