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.
As an ex-employee you may owe a duty of trust to your ex-employer under your contract, for example preventing you from disclosing trade secrets.
