Apr.10 (GMM) Nico Rosberg has stepped up the pressure on friend and formula one rival Lewis Hamilton to join the drivers' safety-oriented union, the Grand Prix Drivers' Association.



McLaren racer Hamilton, as well as world champion Kimi Raikkonen, Adrian Sutil and Anthony Davidson, are the only active formula one drivers who have resisted calls to join the GPDA.



Following widespread media reportage of Hamilton's stance, however, GPDA chairman Pedro de la Rosa recently insisted that no pressure is being put on the 23-year-old Briton.



But even Rosberg, whose rivalry and friendship with Hamilton dates back to their karting days, would like to see him at their meetings at grands prix.



"What am I supposed to say to him?" the Williams driver is quoted as saying by Motorsport Aktuell.



"He can decide for himself.



"The only thing is that I think it is a pity that there are leading F1 drivers that are not in the GPDA," Rosberg added.