A police spokesman said the injuries were not life-threatening, however.
Lightning struck several trees, then an electrical box at 1pm local time (1100 GMT) near a lake in Dabel, some 70 kilometres (43 miles) from Germany’s north coast.
Emergency services including several helicopters rushed to the scene and treated the injured for shock or burns.
Around 500 people had gathered at the lake to celebrate Father’s Day in Germany, traditionally celebrated on Ascension Day.
In Germany, groups of men tend to celebrate Father’s Day together, often with the help of several beers.