console result

rspec string_calculator.rb
.....F

Failures:

  1) add negative number(s) 
     Failure/Error: raise "negatives not allowed - " + v.to_s if v
     RuntimeError:
       negatives not allowed - -3
     # ./string_calculator.rb:6:in `add'
     # ./string_calculator.rb:53:in `block (3 levels) in <top (required)>'

Finished in 0.00203 seconds
6 examples, 1 failure

Failed examples:

rspec ./string_calculator.rb:53 # add negative number(s) 

string_calculator.rb

require 'rspec/autorun'

class StringCalculator
  def self.add(numbers="")
    v = contains_negatives(numbers)
    raise "negatives not allowed - " + v.to_s if v

    delimiters = /,|\n/
    return add_inject(numbers, delimiters) unless numbers.start_with?('//')

    delim,nums = partition_input(numbers)
    delimiters = delim[-1]
    add_inject(nums,delimiters)
    # implement using split, map, reduce
  end

  def self.partition_input(str)
    str.split("\n")
  end

  def self.add_inject(numbers, delimiters)
    numbers.split(delimiters).map{|n| n.to_i}.reduce(0,:+)
  end

  def self.contains_negatives(numbers)
    /-\d/.match(numbers)
  end
end

describe "add" do
  subject {StringCalculator}
  context "empty string" do
    it {subject.add.should == 0}
  end

  context "one number" do
    it {subject.add("3").should == 3 }
  end

  context "unknown amount of numbers" do
    it {subject.add("4,6,8,10").should == 28}
  end

  context "a newline in the numbers" do
    it {subject.add("1\n2,3").should == 6 }
  end

  context "given a delimiter at the beginning of string" do
    it {subject.add("//;\n1;2").should == 3}
  end

  context "negative number(s)" do
    it { subject.add("-3,3").to raise_error }
  end
end

One other suggestion I would make is to have your subject capture the method you're testing and pass the param as it's defined with a let under each context. You can dry things up considerably this way:

subject { StringCalculator.add(string_of_numbers) }

context 'with one number' do
  let(:string_of_numbers) { "3" }
  it { should == 3 }
end