We don't have many of them, and the use case is a little vague. The Twibbon thought could be accomplished by tracking the number of "followers". Intuitively it seems ot make sense to have a "Members Only" security Capability, but I think the results can be achieved by limiting the editing capability at the content level in an open group.
Two comments. Following is independent of joining altthough there *are* defaults that you follow when you join but the concepts are discrete.
Setting edit rights on content would not give us what we want and most times users wouldn't bother to restrict this setting. Exacerbated by the inability to set the default setting per place to, say, creator only can edit.
No, letting the owner or others see that a group has N members who have participated or contributed is precisely what I like about Members Only - although to be fair this 'position' pre-dates CMR which gives an equivalent feedback to selected users such as Owners, Sys Admins and soon this new category of users with CMR rights.
Note: this question is just me personally asking, not indicative of anything more than curiosity.
Members Only groups have been around since before I joined Jive. I'm a member of some, but I'm not sure I have entirely understood their value prop. Do you use them? If so, when and how? Or, when and why would you recommend to someone that their group should be Members Only?