Theseus met Percival during a student exchange or a Triwizard-like competition or any such collab between Hogwarts and Ilvermony. They became friends and later penpals, until graduation and after. Both pursued a similar career choice as auror-in-training, and a few years down the line, it's Graves' turn to visit Theseus. It can be a personal invitation (maybe ambitious Graves is stressed out because he climbs too high and too fast, and Theseus invites him down for a holiday bcos mate, we haven't met in ages), or it a formal visit to strengthen the diplomatic relationship between MACUSA and the British MoM, or any other reason.

Graves was invited to stay at the Scamanders' house and that's where he met Theseus' younger brother, final-year student at Hogwarts, who was home for holiday. Things ensued :D