(I came across this when I tried to join my test community to try something out for a support request and found that I couldn't, since it was completely Closed rather than Moderated.)
The first section on "MEMBERSHIP" should mention Moderated membership and that only this level allows users to request membership in a community, while Closed membership means that nobody can join the community at all, nor can a maintainer add them. (So it's pretty much useless if you intend people to join the community later, unless you change the setting to let them in.)
Perhaps something like this?
A community's membership may be either open, moderated, or closed. In an open community, any user can go to the Community Info page and join by using a link near the top of the page. In a moderated community, prospective members must click the link, which makes a request to join the community. The maintainer of the community must then approve the user's request to join the community. In a closed community, nobody can join the community.
along with additions to http://www.livejournal.com/support/faqbrowse.bml?faqid=80, which should mention that you shouldn't make your community fully closed if you intend to add users. Perhaps something like this?
When you create a community, you decide whether membership will be open, moderated, or closed, and whether all members will have posting access ( http://www.livejournal.com/support/faqbrowse.bml?faqid=79 ). If later you change your mind about how those options should be set, you can go back to http://www.livejournal.com/community/manage.bml and edit the Settings for the appropriate community.
If you allow members to join, but want to grant posting access individually, you can do this via the Community Management page ( http://www.livejournal.com/community/manage.bml ). Access the "Members" link. Here, you can grant posting access to existing members.
If you've selected to create a moderated community, potential members must request community membership by attempting to join (http://www.livejournal.com/support/faqbrowse.bml?faqid=101). At the bottom of the Community Management page (http://www.livejournal.com/community/manage.bml) you can select to be notified by email every time someone requests membership in a community you control, or you can choose only to be notified when you load the Community Management page.