Sometimes I want to reference someone to an earlier post on the Message Forum but haven't found a way to do that.
Is there something I have missed?

Here's my guess:
When you click on the post you wish to reference, there should be a URL (web address) for that post. For example, the URL for your class is costmesaclassof1963.org. When you reference someone to the post, let them know its URL and that should take them to the post.

Not in the message forums - those are one long single thread guest book style message forum