The word for back is costas. Which appears to be plural, but is not. As in “Ele tem dores nas costas”, meaning he has pain in his back. So would a group of people with sore backs be " Eles têm dorem nas suas costasas" ??
In the learning note about pluralising words, costas is mentioned as one of the exceptions. It is already plural, as your example also demonstrates “ele tem dores nas costas”. So for a group of people it would still be “eles têm dores nas suas costas”.
There is no singular word for it - a bit like the English word “sheep” which is also always plural regardless of whether there happens to be one or a hundred of them.