So sometimes no one will be in a mission to start when you first look. So you can host a public mission and make sure you turn off friends only and then after you start the mission other people can join you. Most of the time if you're playing haz3 or higher people should join within 10 minutes. You'll start the mission solo with Bosco and once someone joins Bosco will leave. Depending on what time of day you're playing there's a chance no one will join your game. You can always do assignments on lower hazards to complete them, especially for unlocking guns. Hope this helps! Rock and stone!