I don't think that that would be a fair usage of the forum. If I'm not mistaken, you are talking about users like
@sfsameer, who sells their source code... although the code is B4X based, they also include Java and other base code libs and API.
They sell a product and as such it's up to them to provide support by their own means.
I am surprised to see that they have not yet created their own forum on their web page.
Creating an invite only system would be ostrasizing users that did not bought their source codes... then imagine that, after a few months or even a couple of years, they decide to make them freeware... what is supposed to happen to those invite only threads that may have "died" long ago??