It is true that there is a sub judice convention that exists that usually says that Parliament shouldn't deal directly with issues that are before the courts. However, there's quite a lot of leeway within that in the sense that it is a convention; it's not a hard and fast rule. It is something where members are advised to be careful when asking questions or when inviting witnesses. The sense is to not have Parliament prejudice a process that's already going on before the courts, in order to maintain the integrity and the independence of both branches of government, the legislative and the judicial.
That said, there are ample examples of committees inviting witnesses who were involved in legal issues. It's really an invitation. It's incumbent upon the witness to decide whether they want to appear or not. I would advise the chair and all members that in dealing with a situation where the sub judice convention is relevant, members should exercise significant caution when asking questions, to ensure that they are not putting the parliamentary process in a situation in which it may have an inadvertent influence on the process that is ongoing on the legal side.
The committee is still well within its powers under the standing orders. If a witness refused, the committee could still issue a summons for them to appear, but that is a last resort. Usually I would suggest that you invite the witness first.
It's difficult for me to say what would happen at this point, because I'm not sure if the witnesses will be forthcoming and wish to appear before the committee. However, when I was considering this motion, I did advise the chair, as I would advise all members, that caution should be used by the committee members when pursuing this, to ensure that there isn't seen to be any sort of infringement on the court's rights.
I hope I've answered your question. Thank you.