If you want more people to join the only way is to spread the word of this events! I always retweet the jam announcement even if my reach is really small, maybe someone new will see it thanks to it. I also try to participate if I have the time even if my skills are that of a newbie, I think it helps showing how the jam accepts all levels of skill, it's also a fun creative challenge so I encourage people to participate, spread the word and maybe we'll get a big number of submission for Jam 100!