It makes use of a BML security hole, so it may suddenly stop working one day. But, you use the web journal entry (it won't work with the clients) and enter:

(=LJCOMM the_community_here LJCOMM=)

... just like the < lj user=... > thing. Submit the entry.

Go back and edit the entry (you'll see that it has replaced the BML with HTML), don't make any changes, but save it again. Voilà. =)

It takes two passes to interpret BML... which means this won't work in the Comments.

Yeah, it is =) Wish they'd just implement the <lj comm=...> and be done with it. =P

Or, you can enter the HTML manually:

<a href="/userinfo.bml?user=the_community"><img src="http://img.livejournal.com/community.gif" WIDTH=17 HEIGHT=17 ALIGN=ABSMIDDLE border=0></a>&nbsp;<b><a href="/users/the_community/">the_community</a></b>