You should put your contact information in your profile (the info I edited out of your question body). Similarly, if someone wants to be contacted outside the site, you can look in their profile.
If you link to the user in question, I'll explain where that contact info lies. For me, look at https://apple.stackexchange.com/users/5472/bmike
The site notification methods are to @user ping them in comments or chat. Both of these require you to gain some reputation so asking here is your only hope of pinging the user without them self-disclosing their contact information or you gaining reputation.