you have to first contact support to put your jam on the front page, post it to indi games, then make it look really nice, once done, make a discord, after which advertise your game jam useing your url link, with this your sure to get 1-2 joins, but rememmber this, people dont want to join a game jam with 0 people in it, which means if you can get a few pals to help out and join with a seperate account you can get the number to like 4 or so and attract a bit more people. hope this helps mate!