Helldivers stole the matchmaking form Warframe/Destiny (more Warframe) and I got thousands of hours in both, but Helldivers made it so obscure that I could not figure out how to find a group. I was placed on one once. Then I had a loading screen, then a hidden loading screen (aka "ships fly next to each other sequence"), then walked up to the person I was apparently in a group with, then got kicked, had the same amount of loading, just backwards. After that, I tried several missions, all without any support by anything. The combat system is utterly useless when alone (which I only was because of matchmaking). I refunded this very undercooked game.

You know what, I am gonna call skill issue.
I get that the “press R to join a group” can be overlooked or that not everyone has the intuition to click on the active missions on a planet (alright, these are currently bugged and do not refresh quick enough so always full).

But all one has to do is to do a quick google search to find out you just open the big holo planet and press R, there are also definitely worse offenders in cryptic/useless UIs.