The simplest fix is to make the comparator function be static:
static int comparator (const Bar & first, const Bar & second);
^^^^^^
When invoking it in Count, its name will be Foo::comparator.
The way you have it now, it does not make sense to be a non-static member function because it does not use any member variables of Foo.
Another option is to make it a non-member function, especially if it makes sense that this comparator might be used by other code besides just Foo.