21st Marines was an organic element of 3d Marine Division. Company I was a component of the regiment's Third Battalion (one of the battalion's three rifle companies). 3d Marine Division didn't participate in the Guadalcanal Campaign but staged and trained there for the assault and seizure of Bougainville and the Campaign in the Northern Solomons.
Third Marine Division Association maintains a website at < http://www.caltrap.com/http://www.caltrap.com/ > and publishes a monthly newsletter. Perhaps you can make contact with former buddies through either or both of the above.