When HMRC wish to litigate a case they normally pick one individual (with a weak case). That individual or more likely the promotor of the scheme or a Firm of Accountants or Lawyers will invite other participants in that scheme to join in a Group Legal Action.
They will be asked to contribute a fee (sometimes known as a Fighting Fund) to finance the usually substantial legal fees involved. If the case progresses through the courts they will be asked to "stump-up" more funds to finance the action with no apparent benefit to themselves. The group participants will be bound by the decision of the court and will have to look to Rule 18 of the Tribunal to distance themselves from the decision at a later date. Many participants of group representation believe (misguidely) that their group is taking the action against HMRC. That is a fallacy. However, the group may apply for a Judicial Review of their case and (as has been seen recently) a review of the validity of Accelerated Payment Notices (APNs).
Ultimately, when a final decision is reached at the end of the core process (which could take many years) all the participants of the scheme will be bound by the final judicial decision irrespective of whether they took part and paid for the group representation.
Bare in mind that group representation does not represent the individual in their personal dispute with HMRC on their own affairs. All advisors will charge separately for this service. If you are thinking of getting involved in this remember that HMRC have deep pockets and will almost always take up the opportunity where permission is give to appeal a decisions which is not in their favour. They have access to leading legal counsel and this is evidenced by the case of Philip Boyle which became a final decision with no appeal by the tax payer at the first tribunal hearing due to the lack of skill by defence council.
REMEMBER, HMRC LITIGATE. THE TAXPAYER WAITS.
These comments are made by a Tax Specialist who is not a lawyer and not qualified to give legal advice. In all such matters, you are strongly advised to take legal advice from someone who is properly qualified in this specialised area of legal practice.