Agadir has an international airport (20 km southeast of Agadir city); Agadir-Al Massira Airport (IATA: AGA) with direct scheduled flights from major cities in Europe as well as many Charter and low cost flights arriving from all over Europe. Taxis can be engaged at ranks at the Airport with a capacity of six passengers for a cost between 25 and 35€
If you are landing in Marrakech-Menara airport, high standard buses from major companies such as CTM (http://www.ctm.ma/) and Supratours (http://www.supratours.ma/en/) connect Marrakech to Agadir on a frequent basis. Buses should be taken from downtown Marrakesh in the main bus station for CTM and in the train station for Supratour.
The trip from Marrakech to Agadir takes 2.5-3 hours, through the scenic Atlas Mountains, and costs approximately 10-15 Euros for a single ticket.