That's what I was thinking as well, but seeing that Kier has to travel from jolly ole England to stand trial in the US to face some coward owned and operated company, I would think that anyone with a vested interested in XenForo Ltd. as a company ( owning a license is an interest I would assume ) should be openly invited as when filing the brief you can be representative of a group.
Only the person, persons, or respective party responsible for filing the claim need be present and by proxy can be representative to a entire groups notions.
an expert witness is applicable if as on the customer end of things you have owned both vB and XF you would be a real life witness who spent your money for both products and has the right to your redress of grievances at least in the US. That being said where else could an expert witness be materialized from. Anyone with business ties to the companies can be excluded as expert witness due to conflict of interest. However as a user of both you could hardly be considered biased because you genuinely trusted both entities at one point and invested in their products and helped secure those companies futures. When one of those companies violates that trust it is your responsibility to voice that to them when they don't listen you take them to court. In the case of vBulletin many users simply wished to abandon ship, in reaction to us finding a place that cares about its customers they opened a case against the creators of another software instead of worrying about their own backyard.
So all jokes aside assuming you owned both, you are a credible witness.