If you've selected to create a closed community, no one is able to join. To allow someone to become a member, you will need to switch the community's membership settings to open or moderated (http://www.livejournal.com/support/faqbrowse.bml?faqid=79). They can then request membership. After they have joined, you can switch the community's membership settings back to closed.
Seeing that community invites are mentioned in the next paragraph, this rather complicated way of getting people to join a closed community could be removed (even though it's possible and it can be done: people don't need it since there are invites) and you could kinda introduce the next paragraph/section
If you've selected to create a closed community, no one is able to join. To allow someone to become a member, you will need to invite them to the community.
INVITING USERS TO YOUR COMMUNITY
You can invite users to your community, which will allow them to accept or decline membership. You can do this whether your community is open, moderated, or closed, but it is most useful for closed communities.
Or something like that. I didn't bother to rephrase things.