Those desiring to contact men who served with relatives or friends during the war should consider the various unit associations. Division and regimental associations are made up of men who served with the division and or regiment during the war. These associations proliferated after the war BUT not every outfit formed an association and, due mainly to the dwindling number of World War II veterans, some of these associations are now defunct - particularly those representing outfits which have been inactive since the war.
An association's title is usually a configuration of the unit's present or former official designation. Most are simple (FIRST MARINE DIVISION ASSOCIATION, 38th INFANTRY DIVISION ASSOCIATION, 30th INFANTRY ASSOCIATION [represnting 30th Infantry Regiment], etc.) while others may be a little more elaborate or pretentious (SOCIETY OF THE 3d INFANTRY DIVISION, NATIONAL ASSOCIATION OF THE 6th INFANTRY DIVISION, 76th INFANTRY DIVISION VETERANS ASSOCIATION, etc.). It's very simple. You contact the association and they put you in touch with living former members of the subordinate unit involved. Many associations have newsletters and are more than happy to run inquiries.
Contact the Reference Desk at your local public library. They MAY be able to help you (1) determine whether an association exists and (2) give you the current address and phone.
VERY IMPORTANT. Many regiments were assigned to different divisions (or major commands) than those to which they were assigned in World War I. Make sure you have not only the correct regiment but the major command also. This may involve some extra work on your part but it's well worth it. These old guys love nothing more than to relate their exeriences and, even with a few embellishments or omissions here and there, you can very often get a good deal more (and better) information than from family or (especially) a government agency.
Almost forgot. Other arms and branches are represented by associations. AMERICAN BATTLESHIP ASSOCIATION is an organization of former sailors and marines who served aboard battleships; other associations represent members of the naval service who served aboard destroyers, carriers, etc. Army Air Corps veterans have a number of associations - usually representing major commands - such as 8th AIR FORCE ASSOCIATION & HISTORICAL SOCIETY. Determine what major command your individual's unit was assigned to and there's a good possibility it's represented by an association.