It is really easy to join, all you have to do is just garden and grow food.
It does not have to be your garden, an allotment will do fine, failing that find some land and use it. Container box gardens also applies, along with window boxes or anywhere or thing else that you can grow food in. Even if you only have access to woodland practice forage management and plant out apple tress and soft fruit bushes, in years to come you will reap the benefit of this investment.
So join up, oh, you already have, excellent work! It may not be the front line but it is just as important.