The MiFI devices we supply allow multiple WiFi connections, from 6 up to 10 devices (depending on the model).
It's possible that the WiFi needs to be reconfigured to allow multiple connections but we'll also need to see how you have congifured both the device and the WiFi devices. Make sure that all devices are set to the correct WiFi encryption method and also that the WiFi network is not hidden (which is a common security setting).
You may need to experiment with removing the WiFi network completely and setting it up from scratch. Start with the lowest security settings and then try to set it to the next level (Open, WPA, WPA2, etc) and see if your devices are perhaps not limited to a particular encryption and cannot read networks on higher bit-rate encryption level.
If you are in the Joburg area, I would recommend popping into our walk-in client service centre and letting one of our experienced consultants walk you through everything I have suggested.