Grit is for things they eat that are NOT commercial feed. If they were fed only commercial feed, they wouldn't need grit. So don't mix it with the feed, but offer it in a separate dish.... if they don't have access to the ground in a run where there are little bits of gravel, rocks, etc. for them to USE as grit.
As per a suggestion from another BYC member (sorry, can't remember who), I nailed an empty, clean cat food can to the wall by the feeder to hold the grit. Works great! As soon as my birds start laying, I'll add another for oyster shell.