But just in case your posts violate the law, remember that whatever you do, you´re never really anonymous on the internet.
This is true.
At any rate, I don't know of an online blog service that would allow for multiple anonymous authors... something like that would probably become a target by spammers really quickly. There are blogs that allow for multiple authors, but you'd have to administrate them (give them a username and password, etc). If you wanted to invest some time and some web-hosting money, you could install Wordpress on your own rented server space and hack it a little bit to allow anyone to post, but you'd still be traceable (by law enforcement) because of your web host, though it would be really tough for a casual or semi-experienced geek to figure out who owns the account. People could easily find out where your hosted by pinging your domain name, but they wouldn't know who you are. You'd probably want to use junk personal information if you bought a domain name too.