differenthistorywiki is the second case of a wiki not having any assigned ability to add or remove groups that I have seen tonight. The first being terribletvshowswiki. I think the issue might be more prevalent, and should be dealt with as soon as possible. It may be helpful to let users know that they can deal with it themselves, but I think it would be better to handle it by ourselves.
I wonder if the issue could be caused by the wikis having special user group configuration due to extensions, but that doesn't make sense.