Yes, NetMeeting usually requires you to use an IP address to connect when connecting over the Internet.
While handing out internal IP addresses always carries risk, and doing so with no firewall can be downright dangerous to the health and welfare of a network, you have several things in your favor that reduce this risk.
First, you are giving a specific address out to a specific customer that hopefully you trust.
Second, since you are using dial-up, you are only vulnerable dring the time you are actually online.
Third, since you are using DHCP, you can set lease duration to a small amount of time, thus probably forcing IP addresses to change on a regular basis.
To reduce the risk even further, you might want to consider the following:
Depending on your needs, set up one or more XP boxes specifically to be used for the video conferencing. You can then enable the built-in firewall in XP to further protect you.
Other solutions:
There is other video conferencing software out there. Run a search on Google using the keywords "video conferencing software" to research the alternatives.
Use a hosted conferencing service such as Webex (www.webex.com). This would essentially alleviate your security concerns (conference participants usa a browser to access a central website to meet). Webex offers both subscription services (if you host a lot of meetings on a regular basis) or pay-as-you-go services (if your meetings occur infrequently).