At the moment, you switch these on from a group. And it is called ‘subscription’. I suggest two changes:
1. The link to switch on notifications on the group home page is called “Notifiactions”. And the link says “Receive e-mail notifications of group forum content”. Or something helpful.
2. You should be able to switch on notifications from the My account -> My notifications page. At the moment, you can switch off existing notifications. It would be useful to add groups of which you are a member but haven’t yet switched on notifications with the option to switch them on.