Anyone already in the group can send invites to new members, so if you're interested in joining, just ask and someone will send you one. There's not really any enforced "required" level of skill, just basically you have to know how to play the game and you have to be willing to listen to your teammates. If you're on this website reading this post, then there's a 99.9% chance that you're good enough to play in these. You might get picked last for teams if nobody knows you as a "good player", but that's no big deal

PUGs are a great way to get your fix of competitive play with higher levels of teamwork, coordinated strategy, communication, and player skill than you'll generally find in public servers. If you're interested in playing Dystopia competitively but you haven't done it before or you don't have a clan to play with, playing in PUGs is a great way to get started. Or if you are in a clan but you don't get enough scrims and matches to sate your competitive appetite, PUGs can pick up the slack. I highly recommend them!