Sorry Swirly.

The Royal Naval Reserve and Royal Naval Volunteer Reserve were the two organisations involved. RNR officers were often merchant navy officers or retired RN officers who would be called up in case of war. RNVR were more often amateur yachtsman and boat enthusiasts. I think that it was entirely up to individuals to volunteer for these services, but I'm sure that the liner companies would encourage their employees to 'join up'. But that's more opinion than fact, perhaps someone else has so real information.