"To join the contest" means to participate or take part in a competition or event where people compete against each other to win a prize or achieve a goal. Full definition
The true value in joining contests like these is not the winning but the community you find there. (booklighteditorial.com)
I was actually joining contest after contest after contest to win a Baby Bond Nursing Cover (and give it away here) before I was given the chance to review it. (thelazymama.net)
Politicker's Hunter Walker reports that Dominican community activist Ruben Dario Vargas has joined the contest for NY - 13. (michaelbenjamin2012.com)