That's the good news. The bad news is that working with multiple contributors increases the administrative complexity of the project, and that complexity will fall on your shoulders. You've gone from "self-publisher" to "publisher" and "editor." You've graduated into the world of herding cats.
You'll save yourself a world of hurt if you make your contributors sign a simple contract, specifying the following:
- The nature of the book: Provide the working title, a description of the reader, what you are trying to do for the reader and how you are changing your reader's life.
- Their task: Tell them how many words you want, when you want it, and in what format. You might give them questions to address in their work, a working title for their chapter, or other suggestions. You might ask for a bio and photo to be used in the book.
- The editing process: Tell them if and when they will be able to see copyedited copies of their work. It is reasonable to show them a copyedited manuscript, though you may not want to give them the absolute right to approve changes. Generally, you should not promise to show contributors a copy of the page proofs.
- Your rights, their rights: Tell them what rights you are buying. It's simplest to buy all rights, but it's friendlier to buy limited rights. This may mean just the right to use their contribution in the book, or on your website, or your blog. In any case, you need to be clear. Your contributors must also warrant that what they write belongs to them and has not come from any other source without permission.
- Their compensation: Spell out whether they are doing the work gratis or being paid in some fashion. Compensation could include money, books, or something else. For small publishing operations, it is reasonable to ask contributors to do the work gratis (especially for limited rights) and perhaps to offer them a copy of the book.
- Their discount: You want your contributors to order books from you, so give them an incentive. For example, the right to buy books from you at 50% off.