Start the Workflow based on User Group Permission using SharePoint Object Model

                SPWeb web = SPContext.Current.Web;
                SPRoleDefinitionBindingCollection usersRoles = web.AllRolesForCurrentUser;
                SPRoleDefinitionCollection roleDefinitions = web.RoleDefinitions;
                SPRoleDefinition roleDefinition = roleDefinitions["Full Control"];
                if (usersRoles.Contains(roleDefinition))
                {
                    StartWorkflow.Visible = true;

                }
                else
                {
                    Response.Redirect(web.Url + "/_layouts/sharepointworkflow/accessdenied.aspx");

                }