Are there any advantages to the server version? I noticed that some versions have an unlimited license. If shared between a group it could drastically lower the cost of the OS. And if it was better in any way, all the better. But I wonder if Apple stipulates it has to be used only in one company.
When you say ONE company, that would depend.
OS X Server can be used for many different things.
Some things such as web hosting and email accounts can be used by multiple people and multiple companies, others can only be used by the computer that has the software installed and any computers directly connected to it such as netboot clients or a classroom network.
There are so many possibilities it would take to long to list them.
If what you are asking is, "Does Apple allow more that one company to use the server and its functions? The answer is Yes, as long as the software is only installed on ONE computer.