Before posting an entry in a community, you will need to join it, as posting access is usually granted with membership
"will" implies that you must absolutely join, while the next sentence says that this is only usually the case. If I read that FAQ knowing nothing, I'd wonder what those unusual cases are, and why I have to join even though there are these cases.
As some communities allow non-members to post, how about
Before posting an entry in a community, you may need to join it, as posting access is usually granted with membership
or using something like "likely", "probably", "possibly" - leaving this to the native speakers?