Hi Steven, on your first question: indeed to be able to follow a private or secrete group you have to be member. following is the way you get notified about the updates in the group. A member can choose how he/she follows a group: via inbox, via self created newsstreams or via the stream following. Or he can decide not to follow, but in that case he/she will not be notified and will have to open the group to get the latest updates.
Can it be that in your second question you refere to the public restricted groups? In a public restricted group everyone can read, comment and ask questions. But to be able to create a blog, upload a doc etc.. they have to be member indeed.
hope these answers help you further.
Here a reference to : Types of groups
The two things - Following and Membership - are two distinct things.
You can only follow a Group to which you have access, which in general means that you can't follow a Private or Secret Group. However, if you choose to Follow a Private / Secret Group while you are a member and are subsequently removed as a member of the Group, you will still be following the group. You won't receive any notifications since you don't have access to the activity or the content, but the Group will still show up in your list of followed places and your name will still show up as a follower of the Group. (This is why it is possible for a Private or Secret Group to have more followers than members.)
Your assessment of the Member Only group is accurate.