A few new details concerning the SharePlay feature have surfaced, following confirmation during Sony's Gamescom conference that it's coming in the 2.00 system update for PS4 later this year.
"How it works is, for example, say, a friend has a game that I don't. I ask them to let me play it, and if they send me an invitation, I can access their PS4 and play while watching the video that is streamed," Yoshida said. "You could call it a mini PlayStation Now."
Yoshida also confirmed to Weekly Famitsu (and via Kotaku) that any game played with a DualShock 4 (so no Play Room games then) will be SharePlay compatible. "There is no need for developers to do anything for their games to be compatible with SharePlay."
Other interesting points highlighted in the interview are the fact the players are able to save locally while using the feature, so if you want to keep progress and buy your own game, you can. Also, as it's a streaming service, you won't be able to use your console while your friend is playing.
Update: According to an update to the linked article, you can't save a game if you're "sharing" another player's PS4.