When granting the
“Domain Users” group rights to Start a process, I encounter a ClientException with
the SourceCode.Workflow.Client.Connection.CreateProcessInstance method. If the
group also has Admin rights, I see the same exception. The only solution is to
give an individual Start process rights instead of for the group.  In
addition, the connection is using ImpersonateUser prior to the
CreateProcessInstance call. Works fine if the individual user is granted rights
instead of as a domain group.

Any ideas why?