On several occasions, I was reading a question thinking "I know a guy that knows the answer to this" and that guy (or gal) also happens to be a registered user. How can I bring this to the attention of this user?
I don't think it's good form to approach users on SO with questions just because they have shown to be knowledgeable in the past. Most people who can competently answer questions here have work to do. SO is a forum for people to pick from and answer questions whenever they are in the mood to, and have the time.
I find it okay to hint at a follow-up question in the comments of an question or answer. Anything beyond that I personally find rude.
Tag the question well so that the Guru of your choice is likely to stumble upon it if they are on SO at the moment.