To add user to role see code below:
//get role from DNN
RoleInfo role = RoleController.Instance.GetRoleByName(PortalSettings.PortalId, DnnRoleName);
if (role != null)
{
//assign user to role
RoleController.Instance.AddUserRole(portalSettings.PortalId, userInfo.UserID, role.RoleID, RoleStatus.Approved, false, Null.NullDate, Null.NullDate);
}

//clear cache
DotNetNuke.Common.Utilities.DataCache.RemoveCache("GetRoles");
DataCache.ClearUserCache(portalSettings.PortalId, userInfo.Username);