5% of Discord users will be lucky enough to be included in the first initial round of testing of video chat capabilities. The team is going to be closely monitoring bandwidth usage, bug, crashes and other issues. As stated in the Discord developer blog, "video data is a behemoth" so it is likely that servers will be under heavy strain during testing.
There is no definitive word on when video chat will be enabled for all users, but devs believe it's at least a month before they can start rolling in more users.
The good news is that testers will be able to try out video calling with anyone, not just others involved in the testing process.
You may want to check your Discord account to see if you've been selected and then head over to the Discord site to learn more.