There are a couple ways to fix this.
A. When you define your Library, use a path to a common location. A location that's identical on everyone's machine--such as the location of a JAR installed with a third-party app into Program Files or /usr/local/ works well or a network drive.
Then, when they check-out the code, the path will still be correct and they do not have to define the Library on their Netbeans workspace.
B. Edit your project.properties file to use a relative path. Open your project.properties file and look for "libs.LIBRARY_NAME.classpath=...
". That will be the "default" location used if the Library is not defined.
Change this to use a path relative to your project and store the jar files in your project. For example:
libs.Log4J.classpath=lib/log4j.jar
Keep in mind that the Library definition in your Library Manager will override this value--so make sure you keep them in-sync (i.e. append a version number to the library name!).
C. Use Vincent's suggestion of using a build-system such as Maven. The Maven build-process will take care of downloading dependencies, etc. Netbeans has plugins for several popular build systems.