Board Thread:Questions and Answers/@comment-190.48.80.135-20140524021440/@comment-24922671-20140524021940

They originally met on the first floor. I'm uncertain if the EXACT circumstances are ever explained, but Argo is the one who originally informed Kirito of that meeting everybody had to go fight the boss on the first floor, in the second episode of the anime.

Argo is only really a character in both the Progressive manga and the Progressive light novels. The Progressive light novels are not yet translated, but the first nine chapters of the manga are. You can start reading it here.