Joining a group is good because
a. It will give you a sense of community. Writing is a solo pursuit and if none of your friends or family are really interested in it, you may have no one to share your neuroses with. So, join a group and find some other film-obsessed people who like writing!
b. It gives you a chance to share your work and get feedback on it. Obviously this has to happen in a fair and equal, non-abusive way. Everyone has to check their guns at the door.
c. It keeps you honest. Being in the group will hopefully lead to you writing more and better scripts.
In short, I recommend it. Has anyone got any tips on screenwriting groups and the running/management/taking part aspects of them? Anyone had some really good speakers at a group? If so, feel free to share!