This page explains how to recruit Grobyc in Chrono Cross: The Radical Dreamers Edition.
Grobyc is a cyborg assassin hired by the Porre military.
How to Recruit
During the mission to rescue Riddel from Viper Manor (Another World), you’ll run into Grobyc as you attempt to escape the manor. After defeating him, he’ll automatically join your party upon leaving the dragon horse stable.