You can't just contact the National Guard and ask for a missing friend or distant relative's contact information. The U.S. military doesn't just give out addresses or phone numbers to anyone who calls up and asks for them. Generally, however, the more information you have on your friend, the easier it will be to make contact.
Use the Locator
The military has a worldwide locator service to handle contact requests like yours. However you'll need information to get information. The Air Force Worldwide Locator, for instance, requires your friend's full name and rank, Social Security number and date of birth. Mail a written request in to the locator office along with a stamped envelope with your friend's name as addressee. The Air National Guard will handle getting the message to them, then it's up to your friend whether to answer. The Army and the Coast Guard do not offer this service, however.
Find the Base
Each state has its own National Guard. if you know which state and branch of the Guard your friend is serving in, you can find the state bases online. In Iowa, for example, the National Guard has bases at Sioux City and Des Moines, and a station at Fort Dodge. Contact the base and ask for a mailing address.
Buddy Finder
The National Guard community page on the military.com website includes a "buddy finder" feature. You can search for contact information on someone with little more than a last name or email address, but the more information you have the better.
Social and Search
National Guard members use social media just like the rest of the world — and a lot of the world is on Facebook. Do a name search to see if your friend turns up. Try other social networks too, such as LinkedIn. If you find the person you're looking for, make contact. If you have friends or relatives who can suggest names of possible contacts, reach out to them online,too.
Another approach is to type the name into a search engine and see what comes up. There's no guarantee this will work, or you may be flooded with false positives, particularly with a common name. Reduce that risk by typing in more information. For instance, you can add "National Guard" to the by-name search or list the specific rank. "Col. Fred Smith" will turn up fewer results than "Fred Smith," for instance.