This error means "This port is alread used"
1. User1 in PC1 starts ipmsg.exe ... OK
2. User2 in PC1 starts ipmsg.exe ... bind error
It requres 2(or more) IPaddress in PC1.
(It can be changed in network adapter settings in network settings in controlpanel)
If PC1 has 192.168.0.100 and 192.168.0.101, then...
1. User1 in PC1 starts ipmsg.exe /NIC 192.168.0.100 ... OK
2. User2 in PC1 starts ipmsg.exe /NIC 192.168.0.101 ... OK