Hahaa oh tagging a member? If you wanna mention another deviant in your comment, type [:][icon][id of that deviant][:], minus the brackets and spacing. Example here:
= [:][icon][Hafiy][:]... ah but the deviant won't be notified (not so sure about this now, maybe dA did some changes already regarding this feature) unless you reply directly to his/her comment, comment on his/her page, or on any of the deviant's artworks and journals. If you wanna send private messages, you can do so by sending 'notes'