Hi Lisa, Marbella is about 67 kms from the Maria Zambrano train station in Malaga. Your clients can travel from Malaga train station to Marbella bus station by taxi or by bus. The bus is cheaper and and there is a good schedule of buses from Malaga bus station to Marbella bus station. You will find the frequency, time and cost of this journey by looking at the timetable of buses to and from Malaga bus station.
Malaga bus station is right next to Malaga's Maria Zambrano train station. It will only take about 5 minutes to pull a suitcase from the platform of the train station to the ticket office at Malaga bus station.
Lisa if you think your clients would prefer a taxi, let me know and I will find out the exact cost of the journey, or let me know if there is any other information you would like. There are more answers to Costa del Sol bus travel questions here.
Best wishes from Marbella and the Costa del Sol! Mike