such terms and conditions of employment shall have the nonwaivable right at any time to present grievances to their employer and to have such grievances adjusted, without the intervention of the collective representative, as long as the adjustment is not inconsistent with the terms of the collective-bargaining agreement then in effect for that employee or group of employees: Provided further , That the collective representative of such employee or group of employees has been given an opportunity to be present at such adjustment. Individual employees who do not affirmatively elect in writing to accept the terms and conditions set out in a collective-bargaining agreement may accept any other terms and conditions of employment mutually acceptable to them and the employer.