Actually, two options are correct:
They allow private messaging. - this is your safest option, as review sites don't offer private messaging (many rewiews are anonymous, so that would kill the point), but all forums do.
You need to sign up to be a member
Actually, this is also true, as many, if not most forums do require you to sign up, so that it is visible whenever a certain person posts again - otherwise you won't know if you're talking to the same person! however, many review sites, for example reviewing doctors, let you post without signing in.