A group of shareholders who make a contract to vote in a certain way
on a certain issue have made a ________?

Dear geckofrog-ga 
The answer is a shareholder voting agreement.

"Shareholders may choose to pool their votes for a particular goal.
Voting agreements may specify that the involved shareholders will vote
their shares together or cooperatively. Courts usually uphold
shareholder voting agreements as long as they relate to voteable
issues."... "Voting pools may specify exactly how the participating
shares should be voted, or they may allow for negotiation and
agreement for each individual issue. Many voting pools include an
alternative dispute resolution procedure for reaching agreement on
such issues."
Further information on 
http://www.bregmanlaw.com/CM/Custom/Custom62.asp?ss=fln-faq-question.xsl
Click on the link ‘What is a shareholders agreement’ at the top of the
page.

"A shareholder voting agreement is an agreement between two or more
shareholders agreeing on how they will vote their shares. "
Source
http://myphliputil.pearsoncmg.com/student/bp_cheeseman_blaw_5/lec_36.html

An example of one such agreement is at 
http://contracts.corporate.findlaw.com/agreements/amazon/shareholder.1996.06.21.html

I hope this answers your question. If it does not, or the answer is
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Thank you
answerfinder


Variations on the words vote shareholders and agreement until 
"shareholder voting agreement"
://www.google.com/search?q=%22shareholder+voting+agreement%22+&hl=en&lr=&ie=UTF-8&oe=UTF-8&start=0&sa=N
"shareholders voting agreement"
://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22shareholders+voting+agreement%22+&btnG=Google+Search

http://contracts.onecle.com/type/53.shtml
Here are more sample voting agreements.