Is there a way to mention/tag a user in a question or answer?
If you just want to mention a user in your question, you should link to their profile.
If you're trying to get a specific user to answer your question, that's not a feature of Stack Overflow. Ask the question of the whole community. If the person is an expert in the technology you're asking about (and an active SO visitor), they probably follow the tag and will find your question. It's entirely up to them whether they answer or not.
If you want the attention of specific member you can look for him in the chat - you can see the active users list here: http://chat.stackoverflow.com/users
Another option is check their profile page, many have website and even email address mentioned there so just use it to contact him/her.
If I understand this correctly, then what you are looking for is to ask/invite some specific users to check your question and answer them.
You will not be able to do that in your question. Such feature is not available in SE sites.
however there is a kind of work-around to get the user's attention to your question. i.e. You can add a comment to your question using
@<username> which will send an alert/notification to that particular user.