A shared human-robot trajectory tracking controller with collision avoidance is designed for a differentially steered uncrewed ground vehicle. A mixed-initiative interaction is considered, in which a linear blending law combines the human commands with the automatic controller using a passive measurement of the human intent. The automatic control input includes a trajectory tracking controller, designed using a Lyapunov-based approach, and a reactive collision avoidance controller, designed using a relaxed control barrier function. The asymptotic stability of the closed-loop shared human-robot system is analytically proven. An experimental demonstration for the proposed shared control was implemented in a crowded indoor environment with static and moving obstacles. The robot’s dimensions are 72 cm x 58 cm x 55 cm (length x width x height). The crowded indoor environment is a 7 m x 15 m room with two doors with a 90 cm width and a 2 m x 20 m corridor. The tracking error was less than 0.03 m, and the robot avoided collision with three static obstacles placed on the reference trajectory and one virtual moving obstacle. The human user can drive the robot away from the reference trajectory when desired, and when he/she relinquishes control, the robot quickly returns to the reference trajectory.