If they've joined the forums, you should be able to locate their username via our forum search feature (located under the forum categories on your right). If they're on the forums you can send them a message via the forum's inbox.
If they haven't joined the forums yet, there isn't any other means currently to contact them. Our apologies if this is the case. The Devs are currently in the process of revamping our community tools to make this process a easier.