No, you don’t have to join a running club. We all run for different reasons, whether it's to lose weight, keep fit or to enter and maybe even win races. But whatever the motivation for lacing up the trainers and hitting the roads and trails, there's no requirement that you have to join a club.
Having said that, many running clubs offer a warm welcome to runners of all abilities. You don't need to be super fit to join! And if a club is just not for you there are more and more running groups popping up. There may even be a Great Run Local near you.