The add-new-users plugin on my site loops infinitely when adding a single user when on a subsite!!! I tried it as an admin on a subsite and as a superadmin on same subsite, but same thing happens. Doesn't happen, however, on the main site when superadmin adds a user the same way.
I will open support so you can see what I mean. It works on main site; then you can check out testsite2's site and you'll see that adding new users infinintely loops!