Hi everyone,
Due to the UI freezes associated with Steam Overlay mentioned in this thread, you may have noticed that we've disabled overlay for everyone for the time being. We hope to have it back soon, although depending on the exact nature of the underlying issue this may take a while. Sorry!
As a side effect of this change, the "Invite Friends" button in the menu no longer does anything. To invite a friend to your game, you can Alt+Tab out of Zero-K and use the drop-down/right-click options on their name in Steam's friends list.
— Histidine