To become a founder or a member of a PMC, you have to upgrade your Headquarter to level 4. Afterwards, you can build the “Radome” (Radarstation).
Open up the PMC menu by clicking on the Company button on the left. To become a PMC member you have to apply for membership first. Send your application via the profile of your chosen PMC. You can find PMCs by searching for them via the PMC search, or simply by clicking on their scores. If you wish, you can also apply for membership with several PMCs at once. After that, you need to wait if or when your membership application will be accepted.
Via the pencil button in the PMC-menu you are able to establish your own Private Military Company.