I’ve installed the plugin from the link you provided and it worked as intended.
I then used this code snippet here so that group members automatically subscribe to group forum as they join the group.
I also had to install this plugin here to manage group members e-mail subscriptions. I set the default to “no e-mail messages” for all groups, so if the group members want also to be informed through e-mail about forum messages, they can configure that as they want.
