Besides Tommy ofcourse, the bandmembers themselves did not do any backing vocals.
Johan Larsson, "6th bandmember of SW" and also replacement bassist for Andreas for all the shows in 2011, did additional backing vocals on 'Mercy Falls' and 'The Great Escape'.
About playing live and handeling the backing vocals Andreas B. said the following:
"We do a fair amount of backing vocals live. It is very hard on our level to get all that across in a live setting, which certainly is one reason why it will never sound as on the album.
But in almost every track we play live there is somewhere between 2-4 harmonies on every chorus plus extra harmony parts here and there, so we certainly do backing vocals live too.
However, noone sings as good as Tommy so of course it will never sound as on the album. But most people hear us more often on the album than live so I am ok with having it that way.
Examples:
'Hide and seek', 'Taint the sky' and 'Edge of my blade' all have four harmonies on all the choruses, as do 'Not an Angel' and 'Paradise'.
Sometimes it is simply to difficult on the guitar for Johan to sing at the same time, some other time it is too difficult for Kyrt or Johnny (I don't sing).
In those cases we have 2-3 harmony parts, like 'Banish the Wicked' or 'Unbreakable' where the chorus on guitar is too odd to sing over.
In addition to that you gotta remember that we have one world class vocalist in the band and not eight. Tommy can only sing one melody at a time live".
I hope this answers your question.
Maybe a nice addition to read:
http://www.ultimatemetal.com/forum/seventh-wonder/435847-backing-vocals-appreciation-thread.html