Multi-user Interactive Walkthrough

Hi, when I was at AEC Build:London a couple of years ago, there was an excellent demo by Calatrava architects showing several users remotely meeting in, and navigating around, a virtual building model. I was wondering how complex it would be to set up something similar? I’m reasonably confident on importing building info and adding a controllable character. The bit that fazes me a little is the remote addition of other users. Has anyone come across a good guide or tutorial for doing something like this? Thanks,

Hello! Please could you guide me on how to import models, build the walkthrough and control a character to navigate the model?