ISDN is digital so there are no modems involved. The ISDN to Ethernet and phone line device is called a Terminal Adapter. Oftentimes that includes a router that allows you to share your ISDN connection with multiple computers. What device are you using to connect the PC to ISDN.
If you already have an ISDN router all you need to do is purchase an Ethernet switch to connect multiple PCs. Sharing is done by the router's NAT function.