As you would no longer be an employee, your former employer would be entitled to bar you from entering its premises in order to speak with your former colleagues.
Of course, outside of its premises, your former employer cannot prevent you meeting your ex-colleagues. Bear in mind though that, as current employees, they have a duty of trust and confidence which prohibits them from disclosing confidential matters and trade secrets.
In addition, when you left your job, you may have signed a document called a 'settlement agreement' or a 'compromise agreement', under which you ended your employment on agreed terms. This usually includes the payment of a sum of money by the employer in return for promising not to breach the terms of that agreement. These agreements often require ex-employees to promise to keep the amount of the settlement sum, as well as other aspects of the agreement, a secret from their ex-colleagues.
It is very important not to share information with your ex-colleagues in breach of any agreement you have made with your employer. Otherwise, you may find yourself asked to pay back your compensation.