Hiya! Those PVP clans are just groups of players who don't kill each other and ally with other clans/players - no commands necessary. You could start a thread and ask people to join your clan. Hope that helps!
A PVP clan is a group of players who hang out and fight together. It is quite simple to start a clan-I suggest you say in game "I'm starting a new clan, the ____!" (Keep your names original! It can be annoying to leaders of the already formed clans to see a 2.0 version of their clan, without permission.) and creating a thread on the Arenas Forum about your new clan, with an application for members. To make it huge like Pyros--Keep in mind MJB6 has been here for quite some time, and Pyros has grown over the time (Years? lol). It will take a while for your clan to become giant. Patience is the key.
Yeah. This is one of the fun things about clans: You can have fun with other players, and make friendships that extend beyond the Arenas server. Be creative. Try doing things in other worlds on the Sandlot.
Generally ask around, and people may join. I'd love to help you build a base in creative, if you'd like.