lj_everywhere is about LiveJournal adoption by the world outside livejournal.com -- it's both for people who want to run their own servers and people who want to promote outside use of LiveJournal through technical work and evangelism. It has some overlap with lj_dev and lj_biz.
lj_userdoc is about end-user documentation for LiveJournal. I'll be posting chapters of my quickstart there for comments and suggestions, and I'd expect it to be used for discussion of the Guides and maybe FAQs as well. It'll soon have a companion community discussing automated documentation systems and technical reference documentation.
They're both open. Feel free to join.