Retiredwell, if, as you seem to indicate quite strongly, and you do give the impression of being in the 'know' about these matters, that this pressure group is going to emerge as a new political party exclusively for the left of the party who were in the Corbyn court, then I personally believe they should come clean about that now. It does matter to people whether they are signing up for a political party or a pressure group. I would have preferred to see this launch done, without having to sign up, on a platform such as YouTube where anyone could listen to it. I don't like my data out there until I know it is an organisation I want to be associated with. Many members in the Party are to the left, but not in the Corbyn mould. I expect many of those will remain in the Labour Party.
It is quite usual, with their connections and influence, for retired leaders to take on projects but they don't normally try to replace their own Party. If the real purpose is a new party they should have the guts to nail their colours to the mast from the very start.