The second paragraph needs a period at the end of that sentence.
In the third paragraph: You can add any friends that you want to it, and this will allow you to
(awkward split infinitive change)
In the fourth paragraph, would it be a good idea to mention that the URLs to the custom pages aren't case-sensitive? The wording in this paragraph takes advantage of that, so it might be good to mention explicitly since many URLs are case-sensitive.
In the sixth paragraph: "...you may need to close your client and open it again, in order for it to list your newly created custom groups."
The comma there isn't needed.
"The client for Windows" needs a nomenclature change.
In the second paragraph under "Security Notes": "The effect of this is that the entry becomes private (but it will still show the lock icon instead of the eye icon)."
It seems like this might be better stated as "The effect of this is that no one but you has permissions to view this entry, so even though it is not marked private, it is effectively private."
Parallel to this:
"If you wish to make the entry non-private, you'll have to go back and edit the entry (see http://www.livejournal.com/support/faqbrowse.bml?faqid=2 for more information on editing posts)."
Something like "If you would like to change the security level of the entry or set it to a new custom group..."