This is a very interesting point, This bugs me often when I visit a site, see something I'm willing to join in order to reply to or interact with, I join, and then spend 4 times longer than it took to join, Just trying to find the post/page/thing that inspired me to join in the first place.

Essentially one could do what most sites do, capture the referrer of the person being sent to the join form, store it, and if it's ON-SITE we send them back to that page once joined and logged in. But if it's see more off-site, we ignore it and send them to the dashboard.

Most users who have a bit of browsing savvy, will right click join, open in new tab/window, and when finished, simply refresh their original tab, but this doesn't always work on all sites, and isn't an intuitive solution that everyone "Should just be expected to know".