in server/src/jetbrains/buildServer/vsoRooms/notificator/VSONotificator.java [227:234]
private String getUserDisplayName(@Nullable User user) {
if(user == null) return null;
final String vsoUserName = VSOUserProperties.getUserDisplayName(user);
if(vsoUserName != null){
return "@" + vsoUserName;
}
return user.getDescriptiveName();
}