Code:
bella@IdeaPad:~$ id
uid=1000(bella) gid=1000(bella) groups=1000(bella),4(adm),20(dialout),21(fax),24(cdrom),25(floppy),
26(tape),30(dip),44(video),46(plugdev),104(fuse), 108(lpadmin),109(sambashare),123(vboxusers)

su nancy

nancy@IdeaPad:/$ id
uid=1005(nancy) gid=1004(nancy) groups=1004(nancy),4(adm),20(dialout),21(fax),24(cdrom),25(floppy),26(tape),
27(sudo),29(audio),30(dip),44(video), 46(plugdev),104(fuse),122(bind),123(vboxusers),1001(jupiter)

sudo usermod -a -G sudo bella

sudo deluser nancy sudo

sudo usermod -a -G jupiter bella

reboot

bella@IdeaPad:~$ id
uid=1000(bella) gid=1000(bella) groups=1000(bella),4(adm),20(dialout),21(fax),24(cdrom),25(floppy),26(tape),
27(sudo),30(dip),44(video),46(plugdev),104(fuse),108(lpadmin),109(sambashare),123(vboxusers),1001(jupiter)

su nancy

nancy@IdeaPad:/$ id
uid=1005(nancy) gid=1004(nancy) groups=1004(nancy),4(adm),20(dialout),21(fax),24(cdrom),25(floppy),26(tape),
29(audio),30(dip),44(video),46(plugdev),104(fuse),122(bind),123(vboxusers),1001(jupiter)
Test: users and groups / disable nancy / log in to synaptic: Success!!!!

Thanks for pointing me in the right direction!

BTW, I did not add bella to audio because nancy's audio is not working and I added her to the group through the GUI while trying to fix her audio problems. bella' audio is working, including the bleeding edge pulseaudio / HDMI.

But then I ran in to weird errors in Banshee and checked the music files. They were not in the right group so I ran

sudo chown -R bella:bella /home/*

And now something else is broke and I'll need to open a new thread.