Whoever needs to be there should attend. You should consult with your mediator as to what will be the most productive combination of people.
The parties to the dispute are an obvious choice. If they are legally represented then usually lawyers will also attend the mediation.
However, it is important to consider whether there are any third parties who should also attend. For example, is there someone that the party regularly consults e.g. a business partner or family member? If so, it may be better to have them in the room to support the mediating party rather than at the end of a phone line. In addition, it gives them the benefit of hearing all the information that the party is using to make a decision, rather than a second-hand repetition.
On occasion, a supportive third party can work with the mediator to help the party consider the reality of their situation and options.
Or the mediation may relate to a family mediation in which case there may be children or elderly family members whose future are being determined as part of the mediation. There are specific types of mediation structured to include the thoughts and opinions of children (child-inclusive mediation) and the elderly (elder mediation).