It's really good if you are into moba-type gameplay.
Some people will compare it to something like overwatch, but it's really closer to Dota 2 with shooter combat. It's a cool mix of map-control/strategic elements of Dota with more twitchy aspects of arena-ish shooters.
I've really enjoyed it, but then again my two most played games of all time are dota and tf2.
You just need to get invited by someone you are friends with.
It used to be you had to be friends for at least a week, but I think that isn't the case anymore.