We generally approve all requests to join. We would only deny a request if it came from a known vandal or spammer (and of course, we will immediately remove from the team anyone who spams the wiki).
NOTE: This team formerly had a much broader mandate: to improve Launchpad's help material. However, that mandate is now shared across the entire Launchpad community, so discussions about documentation should take place in the usual community forums: the Launchpad Dev mailing list or in the #launchpad-dev IRC channel. The Launchpad Documentation Team team is now just a gateway to wiki edit access (we had some spam and vandalism problems in the wikis, unfortunately, so we had to discontinue open editing).