Anyone can attend if invited by one of the participants. This is what the fair Trading website says:
It is important that all the relevant people involved in the dispute attend the session. Parties may bring a support person as well as a solicitor if you feel it is necessary and will assist the discussion.
However, the same website (CLICK HERE) also stresses the need for confidentiality, so turning up mob-handed or promoting it as a spectacle isn’t going to go down too well. That website has a lot of other valuable advice that makes it well worth a visit.