the sequence is determine by the order of the user in the group , In fact GroupTask do not case about the order of user.
SPGroup group = web.SiteGroups[roleName];
List<SPUser> users = new List<SPUser>();
foreach (SPUser user in group.Users)
if (user.LoginName.ToLower() != "sharepoint\\system")
if you want to control the order of the user , you can use MulitTask , and the Initialized event of MultiTask , you can assign users by sequence , for example:
void GroupTask1_Initialized(object sender, EventArgs e)