All communities have a link at the top of the Community Info page reading "To join this community, click here." When you click this link, one of the following will occur:
* If the community is open, you will be able to confirm your choice to join the community. You will also be asked if you would like to add the community to your friends list (also called "watching"). You can leave the box checked to do so or uncheck it to leave the community off of your friends list. If you change your mind later, you can add or remove the community at any time by modifying your friends list.
* If the community has moderated membership, you will be asked to confirm your choice to join and the community's maintainers will be notified that someone has requested membership. The maintainers can then decide whether or not to add you to the community.
* If the community is closed, you will see a list of maintainers that you can contact for more information about joining.
If you have been invited to a community, you can accept or decline the invitation at
http://www.livejournal.com/manage/invites.bml. You will not receive an e-mail informing you of the invite. Invitations are valid only for 30 days.
Joining a community will usually give you both membership and posting access. Membership is the ability to see Members-only posts, if there are any, and also means that your journal's entries appear on the community's Friends page. Posting access means that you can add an entry to the community's journal.
Eligibility for membership in a community is determined by the owner. There is no other way to join a community, and there are no LiveJournal rules to define what an owner may or may not set as their requirements for membership.
FURTHER READING
Community Info page
http://www.livejournal.com/userinfo.bml?user=communityname, where "communityname" is the username of the community
If I join or watch a community, can the members see the protected entries made in my journal?
http://www.livejournal.com/support/faqbrowse.bml?faqid=81
How do I post in a community?
http://www.livejournal.com/support/faqbrowse.bml?faqid=76
What is "watching"? Can I view communities separately from the rest of my friends list?
http://www.livejournal.com/support/faqbrowse.bml?faqid=179
What is LiveJournal's Friends system? How do I use it?
http://www.livejournal.com/support/faqbrowse.bml?faqid=61.
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