If it's an office, and they are in the same department, they may all be running similar software (even spyware inadvertently shared).
If the access point is the same as the one used by other unaffected users (who can use the wifi), I would look at the similarities in hardware and software on the affected machines. Perhaps all running a different A/V package from the rest of the building.
Can you describe more of the differences between affected and unaffected users? Are the users in the affected portion of the building the most recent to try to get on wifi?
Have you tried moving one of the affected computers to a place in the building where wifi access is okay for others? If it connects, then the problem has to be area specific (but I doubt it -- if the signal is strong, I suspect something else).
The yellow exclamation point (limited connectivity) is usually a problem with DHCP. It could be the DHCP pool is all used up. Might check there. Or if using a domain and they are also connected via a cable, there may be a conflict.