They are for everyone who gets an invite. How selective those are and who gets one depends on the company. The general rule is that the more hireable you are, the more they'll be interested in inviting you. The best way to know about them is to talk to the people at the expo booths. Some are very public, e.g. Google let's everyone in who scans the QR code at their booth in time. On the other hand at GResearch you had to sweet talk the recruiter. I stopped talking to the booth people long ago, so I miss out on most events.