Can my employer ask us not to join a union?

No, employees are prohibited from making an offer to employees which is designed to induce them:

• not to be (or seek to be) a member of an independent trade union
• not to take part in the activities of an independent trade union, in their own time, or in work time where allowed
• not to make use of trade union services, in their own time, or in work time where allowed
• to be or become a member of any union, or of one particular union.