Science fiction and fantasy, romance, westerns, mysteries and horror are the big genre categories of the fiction publishing world. Each genre has one or more organizations that provide representation for professional writers in that particular field. In addition, all of them provide member benefits, many of which can be very helpful to both new and established writers.
'Professional' in this context means that selling a novel or the equivalent is usually the criteria for full membership in that genre's organization. Vanity publications don't count, and depending on the genre, self-publications and e-publications may not be acceptable either. But don't despair if you want to sign up but haven't reached this particular milestone. There are usually different tiers of membership, such as 'affiliate', that extend many of the same benefits to emerging writers.
Multi-genre associations like the National Writers Union are also worth checking out. The union primarily provides representation for journalists and other nonfiction writers but they won't turn you down for writing SF/F. Neither will the Authors Guild, which purports to be the "professional organization for every published writer."
Both include a number of prominent science fiction and fantasy authors among their members, though it's a good idea to remember that these particular groups are working for writers' rights on the grand scale. As such, they may not be very useful for more genre specific and smaller scale issues. For example, the National Writers Union's hard won Tasini decision might make the New York Times think twice about selling your work in e-databases forever without paying you for it, but it doesn't necessarily mean that, say, Bug-eyed Monsters Forever will hesitate to attempt the same rights grab. However, there's no rule saying that you have to belong to just one writers' organization or any at all, for that matter; it all depends on what you want out of your membership.
So why would you want to shell out your hard-earned cash to join one or more of these groups? Let's say that you have a writing contract in hand and you've got some questions about it. Maybe you already have an agent and/or a lawyer but if you want more information, this is a good time to join the writers' organization in your current writing genre. Science Fiction and Fantasy Writers of America (SFWA), for example, can provide a handbook, chat rooms for members, and a quarterly magazine, all with lots of other pros' opinions about what a contract should look like.
If worst should come to worst, it and other organizations like it can also provide access to grievance committees, as well as some legal support if mediation fails. Belonging to a writers' organization can mean that you're not completely on your own if you're asked to sign away all rights in perpetuity for your next 17 works. SFWA, the National Writers Union and the Authors Guild and others use their clout and legal support to negotiate better deals for writers, to force publishers to back away from rights grabs and other useful things that you may want or need to know about.
In addition, these organizations can provide networking opportunities through local chapters, national meetings and conferences, not to mention fun-filled party suites at the big conventions. Once there, you can meet agents and editors and hobnob with the great and near great (and whoever else shows up). Volunteering on committees and at conferences can help you meet people and get your name out. Since the majority of writers' organizations have at most one or two paid staff people, they are very dependent on volunteers to keep things going there (ask not what your writers' organization can do for you...).
Your fellow pros can potentially help you with marketing tips and tricks and other industry information. In addition, you also have access to discounted ads in the group's publications, mailing lists, webpage links, all of which can help you promote your novel. Many of the larger organizations also have either member insurance policies or emergency medical funds or both. If you've just quit or been laid off from the day job, this is something to consider.
Then there is the matter of writing awards. SFWA sponsors the Nebula, Mystery Writers of America sponsors the Edgar and all of the other genre biggies have equivalents. Full members get to vote for who wins as well as being more likely to get nominations. A nomination, particularly one that makes it to the final ballot, can mean a boost in sales to your first or even your midlist novel and make a difference on your next contract.
Along with these more concrete benefits of membership come the less tangible ones. For instance, Horror Writers of America has member-only anthologies, and the SFWA Bulletin and Forum both carry guidelines for anthologies that may not be open to nonmembers or non pros. Full membership can also mean that some editors and agents may take you more seriously because it makes you look more serious about your work and more connected to the genre as a whole.
But don't get too hooked on this part. Being an active SFWA member is no guarantee that your work will be accepted or even read by editors. It doesn't mean that your next novel will sell well or at all, nor does it mean that your fellow members will welcome your falling on their necks at 3 A.M. to weep about the injustice of it all. You will also not automatically acquire the opportunity to meet, date or stalk your favorite writers nor they you simply by signing up.
Another factor to consider before you sign up is group dynamics. Writers' organizations are notorious for an exciting degree of infighting. If you're not careful, it's easy to get drained or discouraged by it. The power struggles, flame wars and so on can also take large amounts of time and energy away from your work if you're in the middle of them.
In addition, writers' organizations tend to reflect their genre's dominant culture. Being part of the first wave of diversity of whatever kind may not be the experience you find most rewarding, particularly if the organization can't or won't meet your needs immediately. Most of the big genre groups also suffer from the degree of elitism that comes with being able to grant the imprimatur of "professional." Even after you've gotten your full membership, there are Pros and there are pros and they aren't always polite about the distinction.
Ultimately, though, the decision of whether or not to join an organization rests with you alone. Signing up is not going to bring instant success but that doesn't mean that it can't be a good and useful thing. Know what you want to get out of membership and know your limits. That said, do some research and check out what organizations do and which one seems best suited to your writing. Take a look at who's in charge and ask for more information if you need it. Once you're signed up, volunteer for committees and activities if you can. It's a great way to meet other members and to give back and to contribute to building an organization that should also be supporting you. And if it's not, remember it's not the only game in town.
Below is some basic information about the main genre writers' organizations. Since this is far from exhaustive, please check their websites for additional information and links to yet more resources, as well as for the nuts and bolts of their membership criteria.
Please visit Writing-World.com's links to Writing Organizations for updated listings of organizations for writers.