Great question, Trisha!
Joining a group lets OTHER people know of your affinity for a group. It shows up on the Members tab, and in your profile. This allows others to potentially connect with you based on a common topic, or even just make an assessment of whether they want to also join and/or follow the group.
"Hey, Trisha is in there, and I really respect her point of view, so I'm going to join, too."
I like how you break down the three ways people can stay updated, btw. One thing I've been telling others is that, even though it seems you get inundated with email notifications from active places, you can DELETE them. It's like an RSS feed, but just in your inbox (or in a folder you set up with a mail rule). The point is that you stay informed, but you don't ever have to keep any of those emails.
Hope this helps! Post again when you start to see some traction from your training materials - I'd love to hear how it's going.
Joining the group provides information to the group managers about who cares about what's going on in the group. I don't know how easy it is for group owners to get that data via analysis of who is getting notices in other ways.
It also, psychologically, forces the user to make at least a minimal commitment to the group.
Finally, it provides information to people considering joining the group as to how many people (and which people) associate themselves with the aims of the group.
Sent from my mobile phone.
With Members Only Groups you must join in order to participate (i.e. "write/comment") in the Group. When I train our users on the difference between this class and Open, I talk about it as really being a part of a community to join it, rather than just being an observer.
The other thing that happens is that when you Join a Group there is a checkbox asking if you want to receive email notifications and the checkbox is checked by default. So, this greatly increases the number of folks opting into email alerts compared to spaces/communities or something like an Open Group would.