While connected to you server, and logged in with the Admin password:
1) Right click in the Ventrilo window.
2) Select Server Admin.
3) Select User editor.
4) Click the user to edit on the left (select "Guest" if your users are connecting as guests).
5) Click the Network tab.
6) Change the Duplicate IPs setting as desired, the default is "1", which means only one user per IP. For instance, set this to "0" to allow unlimited connection from a single ip (not recommended) or bump the number up to 3 or 4.
7) Click the Update button.
8) Click the Close button.