- user's guide
- users guide
- user guide
- users' guide
Although an apostrophe is not incorrect in terms like the following, increasingly it is omitted when the sense of the term is descriptive rather than possessive - that is, when for is meant rather than of.The name of the publication is a little ironic ("Writer's Guide" rather than "Writers Guide").
The implications of their reasoning are also interesting. What is a book for children? A children book?