Cambridge Screenwriters exists to make you into a better screenwriter.
Award-winning writer (with multiple production credits) and Emerson College professor Jared M. Gordon runs a Meetup in which we'll discuss screenwriting practices, answer questions, and workshop your scenes. Whether you've written a dozen screenplays or aspire to, there's a place for you here. Jared also provides more in-depth coverage: http://www.screenplay.guru.
I plan to conduct regular writers' group workshop events, depending on interest. We're only limited by the size of the space wherein we meet, so if anyone knows of a larger venue than those I pick out on here, then please let me know.