[mxuribe]

There are 2 "ways" to do this:

1. You and your friend both use the riot (actually matrix.org) server/network and you both choose any matrix-relevant client (doesn't even have to be the riot client), but only make use of private rooms on that server. This avoids any system setup overhead whatsoever...But the private room(s) that you create would still on a server that is not controlled by you.

2. You can of course setup and run your own little private network; on your own domain name/IP address. This is what I do with my family; and only my wife, and daughter have access (I've even disabled registration). I have not yet connected my little network to the greater matrix network...For 2 reasons: I wanted to beta test this internally so I could leearn; Also, i wanted to be sure my family does not get exposed to any spam (if there is any that is).

Good luck; cheers!