In what it was one of the highlights of Gamescom 2014, Sony revealed during its conference that Share Play is one of the additions that will be included in PS4 system update 2.0. Share play will allow PS Plus members to invite their friends to play a game together even if they don't own the game, however, it appears that these sessions will be limited to 60 minutes.
Despite these sessions having time limit, there's apparently no limit on the number of times players can use Share Play, which means that players can just reconnect and continue what you were playing. Although Sony has yet to state the reason why Share Play has a time limit, we can probably assume that this was done in order to avoid situations like game sharing.
The PS4 system update 2.0, which will add Share Play, Youtube uploads, and more, will be available this Autumn.