Whether you've got a contract with one of the Big Five publishing houses or you're an independent or self published author doing it on your own, you're going to have to pay a lot of attention to your book marketing. Even Stephen King has a social media presence; it's likely you'll need more word-of-mouth than the next King book ever will. The problem happens when writers try to spend so much time marketing they begin to eat into their writing time. There are only so many hours in the day, and you can only multitask so much.
Creating Your Street Team
Beginning writers can't build a team yet; you already have to have a solid fan base for your work. Once you reach the point where you have a few hundred names on your mailing list, and you're getting groups of people posting to you on social media, it's time to build your street team. Put out an invitation in your regular newsletter and post it on Facebook. Set a limit on how many members you think you can handle, but keep in mind that they'll mostly maintain themselves. Some authors take everyone who applies within a certain time period. After the cutoff date, email everyone on your street team and congratulate them for becoming a member. Make them feel special.
What to Do With Your Team
Street teams are the best free advertising you can find. Give each of them an ARC, or Advanced Reader Copy, of your newest book about two weeks before it's published. This gives them time to read the book and get a review ready to post online once it goes live. Encourage them to spread the word, letting people know how much they liked the book. Give team members an inside view of everything you're doing, to give them a feeling of being special and to build excitement. Ask for social media marketing suggestions, and send out videos or special newsletters to give them more material to share.
What Do They Get Out of It?
Besides feeling like an insider with one of their favorite writers, street teams often get unique swag as thank you gifts. You can hold a drawing just among team members and give out t-shirts, coffee mugs, Amazon or Infinity gift cards, and other treats, all with your book's cover design printed on it. Create inexpensive giveaways that tie in with your book, and hand it out to all your team members. Give them special discount rates for larger items, and write special short stories that only street team members will receive. Of course, they'll share the stories with friends, but that's the idea, anyway.
Keep the Faith and May the Force be with You!