The behind the scenes way if you don't feel like downloading a tool -
Login information is saved to ..\PlayOnline\SquareEnix\PlayOnlineViewer\usr\all\login_w.bin
You can login 4 people, save a copy of that file somewhere as like Set1.Bin, repeat for as many sets of 4 as needed, then rotate those files into the login_w.bin position as needed for each set of logins. This part can easily be batched so you don't have to manually copy/paste everytime and stuff.
Code
copy /Y "Path To SetX.bin" "C:\Program Files (x86)\PlayOnline\SquareEnix\PlayOnlineViewer\usr\all"
cd "C:\Program Files (x86)\PlayOnline\SquareEnix\PlayOnlineViewer\usr\all"
del "login_w.bin"
ren "SetX.bin" "login_w.bin"
For logging all the way in, this could be automated with pretty much any programming language but feel that's more something people would build for themselves on a case-by-case basis, rather than make it to share with everyone.