In groups if switching "Members can see participants" a new menu element appears at the left menu to show members.
Using one of "All users can..." no new menu element appears, so there is no place to show members
While this was intended it is arguably why the users within the group are not allowed to see each other while on the other hand the entire world is allowed to see them.
The proposed solution is to automatically select the "Members can..." checkbox when the "All users can..." checkbox is selected.
BUT: I makes still sense that users can manually remove the "Members can..." checkbox if they desire (also no need to change it for existing groups" as the "Members can..." flag also triggers the buddy list feature of the chat.
Some form context help should be added here to explain what the options are good fore.