The site's owners would likely be immune from any civil action based on its users' conduct, under Section 230(c)(1) of the Communications Decency Act:
No provider or user of an interactive computer service shall be treated as the publisher or speaker of any information provided by another information content provider.
The users, though, could still face liability. Although it would be libel rather than slander (because it is published rather than spoken), any damaging falsehoods could be actionable. The emotional distress component would also be a possibility, but it would probably be a tougher case. The laws vary from jurisdiction to jurisdiction, but generally speaking an emotional-distress case requires really outrageous behavior and and very serious emotional damage.