A powertrain system includes a geartrain configured to transfer mechanical power between an internal combustion engine, torque machines and a driveline by activation of a synchronous engagement clutch. A method includes commanding activation of the synchronous engagement clutch, including commanding operations of the engine and the first and second torque machines to control members of the clutch, including monitoring speeds thereof. A magnitude of clutch slip is determined. A zero-slip feedback control routine is executed when the clutch slip approaches a zero-slip state, wherein the zero-slip feedback control routine is operative to control the engine and the first and second torque machines to control the rotational speeds of the opposed first and second members to achieve zero clutch slip. The synchronous engagement clutch is activated when the clutch slip is less than a threshold level associated with the zero-slip state.