If you want to tag a user in a post* and you are in the advanced reply mode, you can use the tag menu icon and search for a user using a dynamic drop down list e.g. Type "p" and all users beginning with "p" will be shown.
However, if you are using a mobile device, you may not have access to the advanced reply mode icons. In that case, you can use the @ symbol e.g. paulwUK will work
* tagging users doesn't appear to notify tagged user at the moment. We are investigating.
Update!! Tagging is a good way to let other users know someone is alerting them to something or asking something. An alternative way to do this is to use alt + click on username. That'll add a tag to your reply instantly