I have heard that this is a big problem with composite decking. From what I have read, a grounding grid or other type of static drain wire doesn’t seem to help since the deck material itself is non conductive.
Some of the composite decking manufacturers claim that the static issue will subside as the decking ages, but it appears that in the meantime, their only answer to the problem is to either to avoid walking on the deck with bare feet, which seems ridiculous and unrealistic to me, especially in the summer months, or spraying the deck with water before use, which IMO, is equally ridiculous and impractical.
I have read that using an anti-static spray coating, something like the one in the link below can help, but it is not permanent and has to be reapplied every so often, so might get expensive after a while.
Staticide 
