You can (for free) set up a livejournal account that allows you to post as a member. (It's like setting up a blog, but you don't have to actually KEEP the blog, you can just use it to join different livejournal communities). Or, you can just comment on review posts anonymously.
It doesn't work like all those forums out there, with little folders for discussion threads. It's more like a group blog, where all members can post reviews for all to see, of their latest teen book reads. And there's a book-a-month that everyone reads. Fun. If you go there right now you can read my review of the new book by Lola Douglas.