I have a network license for Prism Mac. Can this be shared by some users who use OS9 and others who use OS X?

Yes, this is possible with Prism 4, but follow this extra steps below to avoid problems. Prism 5 requires OSX, and will not run under the older OS9.

1. Quit Prism on all the computers on the network.
2. Locate and select the Prism program file.
3. Click Command-I to open File Info. (Or choose Get Info from the File menu.)
4. Check the option "Locked".