Seeing as how IPX was more or less a front to get their hands on alien technology (organic weapons and similar stuff) it's it's mostly semantics anyway.
There were regular employees, "temps" and consultants/passengers. Some were closer to the inner circle while others had no clue what was really going on.
Exactly. There's a difference between being hired by IPX and being in on what they're really after. Having said that, the whole thing about them being a front is a little complicated: -
SPOILER WARNING: For anyone who hasn't read the Psi Corps books
In the first of the Psi Corps books you have them finding Shadow or Vorlon tech (it's a bit ambiguous) fragments out on Syria Planum about a century before they find the full blown shadow vessel and bringing them back the San Diego. Long story short, by the end of if you have Vacit gaining some leverage on the corporation so they're "working for him". So this implies they're in the MRA's (and later on presumably Psi Corps') pocket.
Somewhere along the line Vacit creates Department Sigma and sticks Lyta's granny in charge, which bases itself right out of the Psi Corps Center in Syria Planum (the one Talia spoke to in 'Voice in the Wilderness') and it concerns itself with enhancing teeps, investigating the origins of teeps and hunting down any further traces of the Vorlons or the Shadows.
Where it gets complicated is where does the connection between IPX and EF Bio-Weapons & EF New Technologies come from? I think the answer is with the Psi Corps Director Johnston (a mundane) getting his hooks into Sigma, eliminating a lot of the old school people and putting his own (those raised as normals rather than by the Corps) in strategic positions. Johnston was said early on to be ex-military which probably means Earthforce and after the Minbari War that connection probably meant they got an in with the Corps.
That's probably why Bester was shut out of the Morden/Shadow with Earthdome deal and why he shoehorned Donne onto the Icarus mission and why Earthforce got in bed with IPX and ended up developing Shadow Hybrids.