Good news everyone, I return with updates.
First I have a manga that features quite frequent Group/Multiple TG:
http://mangafox.me/manga/ame_nochi_hare/
which is also one that I'm sure most of you have already heard of, but it counts technically so I'll include it.
The second is less well known and is Irrational Girl Chapter 4 by Ameshoo:
http://exhentai.org/g/682199/28a0c16d83/
Chapter 4 starts on page 35 and features multiple boys (with simplistic faces) in a swimming class getting turned into girls (that all look alike) and then sex happens (as always in hentai).
The series itself does feature more TG, but this is the only chapter that applies to the thread. I'll shortly be searching the ex-hentai gender bender tag and listing anymore that I find.