Tagging people isn't like one would find on Facebook or Twitter or other social media networks. They don't work in the threads/posts. You can tag names on your initial thread like when you tagged it:
Forums: a2k, Forumhelp
But I'm sure that's not what you actually want. The person tagged is not likely to get a notification when the thread is tagged. Only a rare few people like George get threads tagged and when they do ... it's because they requesting English to Latin translations (or similar Latin grammar and definition questions).