Celtx was the very first screenwriting software I used. Back in 2014, Celtx was the golden standard for screenwriting. And in some ways, it retains those features. It's free to make an account, free to work on any screenplay of your choice from start to finish, and free to export your screenplay. Celtx makes formatting easy—and because of the daunting formatting behind screenplays, I imagine the ease of which one can format will be welcoming to any and all beginners. Celtx also lets users physically make a group in-application using the name of the studio, which allows other users to work under that same group. There's also a myriad of data to measure one own's productivity in.
Unfortunately, the major gripe many have with Celtx is its tendency to limit its users. Again, as I have been a member since 2014, I recall when Celtx had a great many more features available to users. Some of my colleagues have made newer free accounts and have surprisingly found themselves limited to only three projects, whereas its competitors—such as WriterDuet—do not have such a limit. And while Celtx may be free, free users may find its functionality severely lacking, to the point where exported scripts have a watermark, making them essentially worthless and unusable in any truly professional setting.
The edits available to the title page is also severely lacking, in what makes Celtx seem years behind competitors. Personally, I love to bolden the title on my screenplays, as do many other produced screenwriters, but this is simply not an option on Celtx for some reason—at least, not to free users. And while competitors may routinely ask free users to subscribe, Celtx makes it a point to beg for the user's subscription every time you sign in. In fact, the pop up the follows after logging it makes it slightly difficult to find the "no thanks" button, as it is almost identical in color to the background it finds itself laid upon.
As I said before, Celtx was once the gold standard for free screenwriting. Now, however, there is nothing in Cetlx you simply won't find in WriterDuet—which, if I may add, does everything better anyway. Consider this: WriterDuet has many, many more features than Cetlx—for $11.99 a month. And that's if one chooses to subscribe to WriterDuet Pro (the free version has more features than Cetlx by default as well). Celtx's pro subscription is $19.99 a month. Choose wisely.
Consider the ease with which all the writers on your production can be accounted for: not only can you visually see them under your company name, you can measure their productivity with a myriad of tools.