To ensure that Mini has been added to your network and can be accessed from your or your friends' computer:
Make sure Mini, your computer, Ethernet router, and hard drive are properly connected (see the first item in this Q&A list).
Make sure that all computers that need to access Mini are either (1) connected directly to the router that Mini connects to, or (2) connected to routers which are directly connected to the router that Mini connects to.
If the hard drive draws its power solely from a USB connector, then it has no need for external power. This scenario should apply to most 2.5-inch or smaller hard drives. For 3.5-inch or larger drives, you need an external power supply by default.
Note: there should be no USB hub inserted inside the computer-Mini-drive chain.