Recently I was working on an workflow where I was accessing a group from SharePoint 2010 and from there trying to get user's mail ids. I created a group in SharePoint 2010 from UI. Since this group was supposed to hold only users (approvers for workflow), I did not assign any permission to the group. I left all the check boxes unchecked.
Next when you try to get the group from code in workflow, you will get error saying 'Group can not be found'. Below is the code I had written in my VS 2010 workflow.
SPGroup spgApprovers = workflowProperties.Site.RootWeb.Groups["Some Group"];
After some googling, I found that if you have not assigned any permission to the group, it will not show up in the code. So I went back to the group, deleted it and recreated with reader permission.
Hope this will help someone...