Alex,
There is not a "Share" option on the current forum, it's not supported at this time. For the open forums, you could just copy the url link at the top of the page, then paste it into an email. Maybe we can looking into a Forum upgrade later to support this.
Also, many browsers on Windows, Mac, iOS and Android have a "Share" option that may make it easier. IE, Safari and FireFox have varioue Share Options, Chrome has an Email Page Location in the File. Just keep in mind, if it is a closed forum, they won't be able to view it without a login. Good feedback.