As the elderly population is rapidly growing in our society, robot walking helpers are receiving more attention these days. To be practical and safe for use in daily life, an important issue is whether these robot walking helpers can be effectively maneuvered and provide guidance to the users. These concerns become even more important for a passive robot walking helper, as it relies mainly on the user's applied forces to move. In this paper, we propose a motion guidance system for a passive robot walking helper. More specifically, we develop a system to identify user's intentions from applied grip forces. This system consists of a pair of force-sensing grips attached to the handles of the robot walking helper and a learning scheme for the mapping between the measured grip forces and the driving force/torque imposed on the helper. The learning scheme, combined with an assistive strategy extended from our previous work, provides motion guidance to the elders during walking. The feasibility of the proposed system is demonstrated via a series of experiments involving motion assistance with i-Go, a passive robot walking helper developed in our laboratory.