thescgiventoks \
When & @when \

documentclass[letterpaper,12pt]{article}
usepackage[utf8]{inputenc}

title{Test Tokens in Environment}

% Counter for scenarios in this story
newcounter{scenariocnt}
newtoks{scgiventoks} scgiventoks={}
newtoks{scthentoks} scthentoks={}

makeatletter
newenvironment{scenario}[1][no title provided]{%
  % Token registers for givens and thens
  scgiventoks={}
  scthentoks={}
  defgiven##1{scgiventoks=expandafter{thescgiventoks @formatgiven{##1}}ignorespaces}%
  defwhen##1{gdef@when{##1}ignorespaces}%
  def@when{@latex@error{No noexpandwhen provided.}@ehc}%
  defthen##1{scthentoks=expandafter{thescthentoks @formatthen{##1}}ignorespaces}%

  stepcounter{scenariocnt}%
  subsection{Scenario thescenariocnt : #1}%
  begin{tabular}{rp{5.5in}}%
  ignorespaces
}{%
  gdef@given{Given}
  gdef@then{Then}
  letargrowsep=empty
  % if the next two lines are commented out, then the Then tokens get rendered
  % it seems that the presence of a \ empties the token lists.
  thescgiventoks \
  When & @when \
  letargrowsep=empty
  thescthentoks \
  end{tabular}%
}%

newcommand{@formatgiven}[1]{%
  argrowsep % first time empty, second time a \
  @given & #1
  gdefargrowsep{\}% next time
  gdef@given{And}% next time
}

newcommand{@formatthen}[1]{%
  argrowsep % first time empty, second time a \
  @then & #1
  gdefargrowsep{\}% next time
  gdef@then{And}% next time
}
makeatother

begin{document}

section{Minimum Working Example}
I'm trying to create a new environment to allow a scenario to be defined.

The environment can contain exactly one verb+when+ macro, and any number of verb+given+ or
verb+then+ macros. Since the macros can be given in any order, the environment uses token lists
to collect the emph{given} and emph{then} invocations, and the closing block of the environment
outputs them in the desired format.

The output I want from the environment should look like this:
vspace{1em}

begin{tabular}{rp{5.5in}}
  Given & the account balance is $100.00 \
    And & the card is valid \
    And & the machine contains enough money \
  When  & the Account Holder requests $20.00 \
  Then  & the ATM should dispense $20.00 \
   And  & the account balance should be $80.00 \
   And  & the card should be returned \
end{tabular}

begin{scenario}[Account has sufficient funds]
  when{the Account Holder requests $20.00}

  given{the account balance is $100.00}
  then{the ATM should dispense $20.00}
  given{the card is valid}
  then{the account balance should be $80.00}
  given{the machine contains enough money}
  then{the card should be returned}
end{scenario}
end{document}