WWOOFers are guests in your home. As such they are usually happy to eat what you eat. It's part of your responsibility as hosts to provide good wholesome food.
Hosts do well to find out in advance if a WWOOFer has any dietary restrictions and to respect those needs, if you accept their visit request. However, you are under no obligation to provide specialized or extravagant foods and may request that the WWOOFer provide those for themselves if desired.
When new WWOOFers come, make sure to explain to them what food is for general use.