If you mean in Q&As, you don't directly. YA is not a chat room. You have two options:
1. E-mail or IM the user through their profile.
2. Wait until you pick the BA and add comments for each user. Probably only the Best Answer will see it since they only get a notification.
The ONLY exception is that you can add additional details to your question for clarification of the question. Do not call users by name doing this or it will be deemed chat.
