Make sure you enable Sharing and specify what users/groups you want to use to share the file.
What is the error message you are getting when the second user tries to open the file?
If you are sharing the file through FileMaker Pro, then the first user to open the file will be the host(presuming that you have sharing enabled on that file as LeeSnover suggested). Every user after that should access that using File -> Open Remote and find the shared file that way, instead of opening it directly from the disk.
If it is hosted on FileMaker Server, and you are still running into this issue, I would open up your FileMaker Server admin console, click the "Database Server" link on the left side, and make sure that the checkbox labeled "Limit Number of FileMaker Pro Connections" is some number of
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Are you trying to use a single license version of FileMaker? Hosting allows multiple users, but they must each have a valid license. Peer-to-peer hosting does not change the licensing requirements for each copy, just the allowable connection number to the host machine (if all are properly licensed).