There is no provision in standard XenForo for a user to email anybody directly from the board like on the old vBulletin board (except for the Contact BBS Admin link which does send email to the Admins). So for directly contacting another BBS user, it's just via Conversations.
For somebody not on TUGBBS you have to use your own email program. To send a link to an entire thread, right-click (or whatever the Apple equivalent is) on the thread name in the forum's thread list and copy the URL to paste into the email you send.
Brian already mentioned how to get the URL for a specific post instead of the thread as a whole.
As an alternate, I can't speak for all browsers, but I've got to suspect more than my Firefox have the ability to send a link to the page you are viewing at the time via the browser's File menu.
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Edited to add:
Apparently XenForo DOES provide a way to send an email to
Share This Page via YOUR email program, by clicking the email button in the 'Permalink' popup you get when you click on the blue post number button.
Example, see
Post #8
But I was unaware of this because *I* don't see any of those Share This Page buttons.
I still know of no way for one user to send a message to another user other than as a private Conversation.