You have a legal right to be accompanied to a grievance meeting about your employer's failure to meet its obligations to you, either by a fellow worker or by a trade union official.
You have the right to be accompanied by a trade union official even if your employer doesn't recognise trade unions (although your union is not under a duty to provide someone to accompany you). If you are to be accompanied by a trade union official, the official must be trained and certified by the union to act as a worker's companion. Check your contract of employment, as this may specify a broader range of people who can accompany you.
If you want to be accompanied to the meeting, you must make a request to your employer, identifying the person who will be accompanying you. The person accompanying you can address the meeting but cannot answer questions on your behalf (unless this is agreed beforehand).