First of all, Microsoft NeetMeeting by itself is pretty "dangerous" program. So, you should install and configure it very carefully. Check Microsoft WhitePapers or look though the Google for recomendations on safety with NM.
Second, it's not only for NeetMeeting. Do you have ISDN Dial-Up at each computer or it's performed at server. In any case you should consider installing Firewall software for all computers, which have direct Internet connection, and antivirus software for all computers. General statistics says, that if you connect Windows computer without firewall directly to Internet, it's enough 15 minutes to get this computer hacked, infected, or at least crashed.
There shouldn't be any particular problem with IP addresses, since with dial-up you are getting new IP address every time you connect. But if you have only one ISDN connection for 30 users, it may be not enough to do video conferencing. I believe that some not-so-expensive Internet options, like DSL or Cable are available mostly everywhere, so, may be you should consider an upgrade for your connectivity. In this case you may get broadband router for 50-100USD which will provide both, connection sharing and security firewalling.
Good luck,
Michael Shavrov
CCDP, CCNP, CCSP, MCSE W2K, Security+, Sun SCSA, ...