I am currently successfully running 4 VMs that are hosting 4 servers each for a total 4x4 map grid. Just add ?MultiHome=xxx.xxx.xxx.xxx to your command line for each server. The IP will be your internal ip address and will be the same for all servers running on that box (in command prompt type "ipconfig /all" to get it).
As for the map image being grey, you need to fix the following lines in your servergrid.json file:
"MetaWorldURL": "http://INTERNET_PATH_TO_WHERE_CELL_IMAGES_ARE_HOSTED",
"MapImageURL": "http://INTERNET_PATH_TO_WHERE_MapImg.jpg_IS_HOSTED",
You must either find a place to host the map images online or host them yourself.