In Star Trek, when people go into a inactive holodeck it looks like a large room, but not really very big. So, how is it that two or more people can enter an active holodeck program and end up in separate locations that are nowhere near each other? Or, are different people seeing different things?
Imagine a holodeck with one user. That user could start to walk, but the holodeck technology must prevent him from walking into a wall. The comment by S. Fruggeiro links to a quote from the TNG Technical Manual, which uses "substrate forcefields" as technobabble:
Substrate forcefield creates "treadmill" effect, permitting participant to remain stationary while the simulated environment "scrolls" by, within the limits of the simulation program.
Also, the holodeck produces visual effects to make the blank walls of the holodeck invisible.
Using this technology, the holodeck could easily put different users into their own personal space if the users split up. They seem to walk away from each other, but they're still only a few metres apart. They can't see each other because of the holograms.
This scheme could be overloaded if a large number of people enters the holodeck, but then the decks we see on the Enterprise are quite large.