In my Moodle I have added a button "Subscribe all users" to the page listing current subscribers to a forum. This gives me an easy way to add all users in a course to a forum. Note that this is different from turning on the option "Everyone is subscribed to this forum" because in my case users are allowed to unenroll themselves from the forum if they wish. There are always one or two students who complain about being force-subscribed to a forum. So I want to take away their ammunition.
Experience shows that while students are very slow to subscribe themselves to forums, they are also slow to unsubscribe themselves once they are subscribed. So this new button achieves the objective of involving all students in a forum without forcing them.
If this makes sense to others, I will contribute the code.