You need to use the advanced editor screen when composing a forum post. Right after the BULLETS tools and just before the MEDIA tools you will see the CODE, QUOTE and TAG USER tools, grouped together. Click the TAG USER tool and start typing in the name. It will insert a link that is supposed to notify the user they have been tagged... @Evie
... Can you confirm this feature is working? I sent in a ticket about this because we aren't seeing notifications when we tag our members in a post.