We currently don't have any guidelines on when Groups should be set to "Moderated - membership requests must be approved." On our Guidelines for Forming New Drupal Groups page it just says "Almost all groups should be open. Use other choices only when there is a good reason for doing so."
We currently have about 30 groups that require approval. That's 30 groups (mostly regional) that potentially stop people from contributing for no good reason. Some organizers have created their group, set it to 'closed' and left, never to be heard from again. And some may have set their group to close simply because the option was there. Whatever the reason, a closed group without an organizer at least approving memberships doesn't serve the Drupal community at all.
I propose we do one of three things; either:
Require any group that chooses moderator approval to have at least 3 group organizers.
Require all regional groups to be open, and require working groups and universities to have at least 3 organizers.
No groups are closed.
"This is where it gets complicated."
I can understand universities being for alumni/employees only. (I don't agree, but I understand.) I don't understand a working group would be members only. But I don't believe a regional group should ever be for members only.
Last September greggles did approve a group for member-only status, however, that group is now open. And the Vietnam group seems to be doing really well as a closed group. But I would say that's the exception to the rule.
Also, I picked "3" because it gives groups a harder threshold to meet, and if one organizer ungraciously leaves, there would still be 2 to pick up the slack.
(And if this seems like a repeat, it is.)
The list of closed groups is in this issue. If action needs to be taken, we can work off that issue.