I'm guessing the error says "/dev/com7" (not usb7). (It doesn't really matter, but I was initially confused by the usb7).
The problem is most likely that the port really is in use (only one program can use the serial port at a time). Are these computers running any firewalls (some block access to serial ports)? Or any PDA sync applications (which sometimes grab serial ports because they might be PDAs)? Do you have Max/MSP or Processing or another program running with which you are attempting to talk to the Arduino boards?
Also, see: http://www.arduino.cc/en/Guide/Troubleshooting#toc2
for other possibilities I may have forgotten.