The Nuclear Apocalypse was pretty sudden, so 'the grounders' on the Ark were pretty much people who were already in space. JR did tweet there were a few shuttles that did manage to get out in time, but those people made up the minority of the Ark's population.
We don't know who any of those people are, but some guesses:
Similar to today's astronauts & cosmonauts – which is about half military, half scientists. These positions are extremely selective and decided by governments. Their pay is middle-class, so in that sense they're regular folks. However, they're all highly educated and trained, so in that sense, they're 'elite'.
Private sector space travel – we already know about Polaris and Eligius Corporation, so we can guess that at least some of the poeple in space work there for private companies (unlike astronauts/cosmonauts who're government employees). These will likely be highly paid and highly educated scientists. However there may also be support staff who're not as highly paid or educated. Depending on how big the stations are, there may be positions like janitors, cooks, secretaries, etc.
Space tourists – there's already been a few of these, so by 2052, there'll be a lot more. However, unless space travel gets cheap, these people would be really wealthy, aka the elite.
Escapees from Earth – these would likely be those who're lucky or really rich. Lucky, as in you're a space pilot and can steal a rocket. Rich as in you own a rocket or have connections to quickly get to one.