1. Why does this community exist? Doesn't the main community make this redundant?
While members are still welcome to post to the main Hetalia community, that comm is so large that having all the doujinshi in one place is useful. It just makes it more convenient when one wants a doujinshi fix. This community is also here for information about doujinshi circles, buying doujin, etc.
2. Why is sharing doujinshi outside of the community discouraged?
Back in the earlier days of the Hetalia community, there was debate over whether or not fanart, such as doujinshi, should be scanned and shared. It was reported from some Japanese doujinshi artists that they mind a lot less when their doujinshi is shared in a locked community of fans rather than posted publicly. We share doujinshi because it can be expensive and difficult for fans who don't live in Japan to obtain, but we would also like to keep it as much among the fans as possible.
3. How can I buy doujinshi?
You can use the sites that were linked in the previous post, or you can bid on Hetalia doujinshi from Yahoo!Japan Auctions. This is by far your best bet if you're looking for a rarer pairing or circle, and you can often get doujinshi for fairly cheap here. If you don't read Japanese, then you'll need to use a service such as Celga to do the bidding for you. However, the combination of international shipping fees and the fees for the service can be quite expensive, so be sure you take those into account before bidding.
4. I can't see the doujin entries from the archive list! What's wrong?
You're not a member of hetalia. You need to join it before seeing the locked entries.
This is just a basic start, if you have questions, please ask and we'll add them! ♥