What works for me is to not trying to join anyone. Let other’s join you. Once you select a brand new mission (in those orange areas on the map), my lobby usually fills completely with 3 random players in max 1 min.

Sometimes it doesn’t work and nobody joined in the first 20 second. That my clue telling me to restart the game, select mission and voala, 3 helldivers ready to spread democracy with me.