Yes and no. Simultaneous logins has been a feature of Unix since the very beginning some 30-40 years ago now, so if you only want to connect to the Darwin half of Mac OS X, and use command line tools to compile your program into a binary for testing on an actual iOS device, then you shouldn't have any real problems.
At least pre-10.7, this sort of thing was not possible with the Aqua GUI (which is what most people think of as Mac OS X), but after a quick bit of googling I have seen some indications it MIGHT be possible with 10.7, but I haven't seen anything official from Apple's site on it. So, you may want to do a little poking around of your own.
Another possibility, though I have no idea how legal it would be, and it'd probably stress a Mini server considerably, would be to install 5 copies of 10.7 into 5 different VMs, and then set each VM with a specific internal IP address, and each of you can connect to a different VM. This would probably have some logistical and legal hurdles to clear, though at least legally speaking since you're not distributing copies of the OS, and it's all installed on a single unit... It would at least muddy the waters a little.
We are a group of 5 developers. We are currently using windows for
programming. But we are about to develop iphone applications. So we
pooled up money and decided to buy a mac mini with lion server.Can
we(5 developers) work with lion server concurrently using our windows
machine? If so, what is the cheapest solution for this?

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