People from Github help contacted and explained the situation.
Since Github changed it’s payment system from per-repository to per-user , it is not possible to have unknown number of collabs on a private repository.
I was also informed that they don’t pressure per-repository users to switch their system(which is good), i wonder why they don’t offer this to new customers. It would be a good thing to be able to buy what is already available.
If you have a similar need, best thing to do is have a developer account and create a private repo. You may invite unpaid users to your repo without a limitation. You will not be able to define permissions on user level though.