This page explains how to recruit Sneff in Chrono Cross: The Radical Dreamers Edition.
Sneff is a stage magician who performs aboard the S.S. Zelbess.
How to Recruit
You can recruit Sneff after defeating the Sage of Marbule at the S.S. Zelbess. Return to the casino on the upper floor, where you gambled against Fargo. You’ll stumble upon Sneff finally winning it big, now that the game’s no longer rigged.
After Sneff leaves the casino, go down to the lower level and enter the backstage room (where you reverted your cat forms). Sneff and his buddies will have a teary farewell, although Sneff ultimately decides to postpone his departure. However, he will offer to join your party.