Sorry, my bad for not clarifying...
Both groups need to be able to edit the document. So "READ" permissions unfortunately won't work.
Hi Chris,
One option is to modify the Start workflow page and show the Start button to those that belong to Contributor group. You can customize with Infopath or Nintex Forms, and this would be feasible.
With InfoPath you need to create data connection to UserGroup web service. See this reference.
If you want to do so with Nintex Forms, then there is an inbuilt function to check the membership of the group.
Another option is to hide the Workflow menu item for the Sharepoint group but this would be lot of work. This requires custom code. Hope this helps.
The Start Form is an interesting Idea. I will give it a shot and mark as Correct once I verify.
Thanks!
Unfortunately, our policies will make it very difficult for me to implement a change to the default start workflow page, but I definitely could see how it would work by changing the start page.