The system knows about other apps installed on your server, but not necessarily apps on other servers. To connect to something on another server you will usually want to obtain a Cap'n Proto capability to it. You might do this through, say, a messaging app that has the ability to embed capabilities. Your friend sends you a message with a capability to some object on their server, and then your messaging app publishes that capability on the receiving server, such that it will now appear in the powerbox for other apps on that server.

Alternatively (less cool, but more practical), you might just drop a URL into the Powerbox and Sandstorm will connect to it and turn it into a capability.