As mentioned elsewhere, Plotbot keeps track of everything written in and about your screenplay.

EVERYTHING.

Along the right side, you'll see a name and time attributed to each element, telling you who wrote it when. Underneath that, you'll see how many versions of the element there are, if more than one version exists.

Clicking on this will show you every revision made to the element, and by whom. From here, you can select a previous version of the element to restore it to the screenplay. You can also revert previously deleted elements this way - again, nothing is lost.
Commenting

You can leave and respond to comments about the screenplay as a whole, an individual scene, or even each individual element - go ahead and let the group know that you think the second interpretation of GUY WITH BIG TEETH's dialogue was better than the third.