Actually it's a two step procedure, first you add virtual USB controller to your virtual machine, then the USB device.
When you add an USB device to your VM you will be prompted to choose between all USB devices connected to physical ESXi host and not already used by other VMs.
Obviously a physical USB device can be used -or "mounted if you prefer- by only one VM at a time.
For more informations about USB passthrough and limitations such as maximum number of supported devices per host have a look at:
Regards,
Paolo