Facebook unveiled a new “groups” feature on Wednesday that has a major problem: Anyone can add anybody to any group they want. So, for example, if you think it would be funny to put your friend in a group called NAMBLA, you can go ahead and do so. In fact, someone did just that to bloggers Mike Arrington and Jason Calacanis. In addition to raising libel issues, BusinessInsider says it could have another unwanted effect: tons of email spam. People can simply create groups to promote whatever they want, and everyone they add will get a message. The solution, BusinessInsider says, is for Facebook to invite people to groups, not to simply add them.