1 Nakama (warning: TV Tropes link).  I gather the Australian term "mates" is similar.

2

The problem with that they themselves mention: that isn't what nakama really means.

The distinction between tomodachi ("friends") and nakama is interesting and subtle. Nakama would be translated as "comrades" except that word has been permanently polluted in English by its use by Communists. I think the best unpolluted English translation is "team mates".

Nakama doesn't necessarily imply the kind of close-knit relationship I'm talking about (though it also doesn't exclude it). That's why I was using sorority and/or sisterhood.

3 Anime seems to display young boys as being more competitive than girls (which isn't true), but for that reason the comradeship that makes Strike Witches enjoyable could easily be ruined if it was a group of boys.

In regards to adding a single boy to the cast: I have thought through that before with the K-ON! cast.  The show would have been something completely different, showing sides of the characters we really don't want to see; such as the girls being jealous of each other or groveling before the boy- if it did become a harem-ish story.
But with the way Miyafuji is presented I don't think it would even work.  She is way too naive and her accomplishments would be overshadowed by the boy in the story.

While these are a couple of reasons why I don't think it would work, writers have pulled off way stranger things with anime.

4

There was a show whose name I can't remember (I didn't watch it) about a girl in the military who is assigned to an all-girls unit and learns to play the trumpet. I gather that it has the same kind of sisterhood dynamic going, and would probably have been ruined if a guy had been added to the cast.

While I was writing this, I found myself thinking about Sakura Wars. The basis of that is a game, and the game basically is a harem. (Though it's not a seduction sim.) Ogami is the player avatar, commander of the unit, and he leads them into combat as missions come up in the game.

The combat effectiveness of the girls is a function of how much they like him. Between missions he has to do social things with all the girls, and make sure not to concentrate too heavily on any one (and avoid jealousy and such like), because if he doesn't they won't fight well, and the combat missions won't be successful.

The fourth episode of the first OVA was about that, and it included some scenes of him doing things with the girls. What I remember is him having a tea party with Iris and her stuffed animals, and helping Kanna take care of a bunch of crying babies. This isn't really the same as a dating sim, and certainly not like an eroge.

The TV series did manage to change that dynamic quite a lot. Ogami as a character doesn't even join the unit for several episodes, and the concentration is primarily on Sakura and her relationship with the other members of the unit -- especially with Iris.

Even so, Ogami is a major character and his presence in the show does change the chemistry quite a lot, shifting it away from that feeling of sorority I prize in Strike Witches.

That, plus the fact that the members of the team don't have quite the things in common that the witches do. What they have is strong ki, but except for Iris it hasn't really affected their lives before that point.

(Nanoha Strikers also managed to avoid becoming a harem, but then, Erio is only 10 years old.)

5 The show was So-Ra-No-Wo-To.

6 The crying babies were a reference to an incredibly annoying mini-game found in one of the Sakura Wars games. Most of them I bulled my way through, to get a good score (and the attendant stat increase with the girl-of-the-episode), but that one I just quit and lived with a ho-hum Kanna in the combat scene. Was. Not. Entertaining.

If you couldn't add an Ohgami, could you have added a Yoneda?

7 That would have changed the Wilcke character rather substantially.

8 Of course, StrikerS managed that by putting the entire male cast (plus Arf) on a bus... I have to wonder if they'll even make cameos in the next one.

I've heard stories that that was done because of the demands of the horde of N/F slash fans and shippers, but I also have to wonder if the writers didn't also just flat-out have trouble figuring out how the guys were supposed to relate to the girls as adults, especially given that all of the relationship stuff would have to have been crammed in between the training episodes, the flesh-out-the-new-characters episodes, and the actual A-plot episodes.

Are there any other series where this gradual kick-the-boys-out-of-the-treehouse thing occurs?  I don't think Tenchi counts, because he remains the focus, even when he's not really contributing.  Saki, maybe?  I haven't seen it, but I think you mentioned once that there was like one guy, who was mostly ignored.

For that matter, does the reverse ever happen?  Would DBZ count, or is that more a matter of rampant power escalation making all kinds of people irrelevant?  And, are there any series that do a proper "band of brothers AND sisters" straight up, without falling into a harem or romance?

9

(Nanoha Strikers also managed to avoid becoming a harem, but then, Erio is only 10 years old.)

That didn't stop Negi, heh.

Are there any other series where this gradual kick-the-boys-out-of-the-treehouse thing occurs?

Bamboo Blade, for the most part. Even lampshaded by the boys.

And, are there any series that do a proper "band of brothers AND sisters" straight up, without falling into a harem or romance?

Slayers? I recall just enough "romance" to permit flat-chest jokes about Lina.

10 The band without any romance is probably too much to ask, there was some even in Samurai 7. Among non-harems, IMHO Dai-Guard and MS 08th Team are the best at treating the nakama issue. Coincidentially both are mecha shows. You'd expect some sword-and-sorcery tactics anime to upstage them, but nope.

11

There's one guy in Saki but he amounts to a plot device. He was there in order to introduce Saki to the club, and later his main purpose is to be confused so that other people can explain things to him (and the audience). They make him a comedy figure just to keep him interesting, but he isn't an important part of the story, and that show was never a harem.

There is a subcurrent of romance in Slayers but little or no angst and essentially no jealousy. Lina and Gourry are in love with one another but neither of them is willing to admit it. Amelia is in love with Zelgadis but he doesn't really realize it. And she's patient, and happy to just be able to spend time with him. Those things do actually drive plot in certain cases, but not into classic ruts.

I think that the guys play a bigger role in the Ikki Tousen franchise in the first series than in any of the others, and as time goes on they matter less and less -- though their significance never declines all the way to zero.

By the end of Dragon Destiny, it's been proven (the hard way) that the most powerful fighters in the city are girls. Sousou was the only guy who was arguably strong enough to even challenge the strongest girls, and he got his ass handed to him by Hakufu and Gentoku. The spirit possessing him was destroyed, and he may even have lost his dragon, though I'm not sure about that part.

By Xtreme Xecutor I think that Koukin was the only guy who had a significant role.

12 Tactical Roar is an example of "one guy in a crew of girls". I didn't get far enough into it to tell if they avoided turning it into a harem show, but I recall that there were already harem vibes by the point where I bagged it.

13 Off the top of my head, what about One Piece & Fairy Tale?  Both are mixed-sex groups, though they have more guys.  (Naruto & Bleach occur to me also, but they're more like DBZ, where the guys dominate)

14 Naruto, ugh ugh ugh. . .  that show utterly fails to show either romance *or* nakama well. . .

15 I think Tactical Roar was going for a relationship with that tsundere crewmember after resolving his former relationship with the captain, rather than a harem. Most of the rest of the crew seemed to be in a harem relationship with the female doctor. Ichigo Marshmallow had a male character with even less of a presence than the one in Saki.

16 How did Mai Otome handle the nakama/sorority aspect? I think there was one would be romantic interest that you mentioned.

17 The situation in Mai Otome is a bit different. The otome all hold primary loyalty to their masters, and they are all aware that they may end up fighting each other to the death if they are ordered to do so. Which means they can't really allow themselves to be too close emotionally.

18 Not to wander from one language to another, but would nakama be similar to the Greek koinonia?  That implies a "kindred-ness" much closer than friendship.

19 Wouldn't know. Let's stick with English and Japanese here, OK?

20 I forgot the ultimate anime band of sisters: Sailor Moon. The Sailor Moon R movie even makes the point about the rest being outcasts before  Sailor Moon. There is a male romantic interest (Tuxedo Kamen) for Sailor Moon, but it doesn't disrupt the relationship the others have. In the final arc of the manga, one of them says that when things calm down, she'll finally get a boyfriend(s). One of the new characters corrects her, saying that she'll always hold Sailor Moon first, which she realizes is true. So there's one way of keeping the sorority aspect.