In 4.5 and prior, joining offers you the option of email alerts, which following does not. But you've got the basic idea - Joining adds you to the member list.
Of course in Members Only groups then Joining is what you have to do in order to contribute. I actually wouldn't be sad to see "Open" groups go away - I don't think it's unreasonable to join in order to participate.
Joining a group makes you automatically follow it : and that’s what you want, to keep posted of the group’s activity in your followed stream. In specific cases, you might choose to unfollow it :
- You still have a quick access to the group (Browse places --> followed)
- In case of private groups, you still have access to the private content if you need it
- But, you are not “bothered” in your activity stream with activity that you don’t want to watch closely.
There are some groups that justify this choice, but it is mostly interesting for private groups in my opinion…
For open groups, I still haven’t figured out why choose “ Follow” or “Join” though ☺